Add the imx8m pcie phy driver and imx8mm pcie support

Refer to the discussion [1] when try to enable i.MX8MM PCIe support,
one standalone PCIe PHY driver should be seperated from i.MX PCIe
driver when enable i.MX8MM PCIe support.

This patch-set adds the standalone PCIe PHY driver suport[1-5], and i.MX8MM
PCIe support[6-8] to have whole view to review this patch-set.

The PCIe works on i.MX8MM EVK board based the the blkctrl power driver
[2] and this patch-set.

[1] https://patchwork.ozlabs.org/project/linux-pci/patch/2021...
[2] https://patchwork.kernel.org/project/linux-arm-kernel/cov...

Main changes v5 --> v6:
- Add "Reviewed-by: Rob Herring <robh@kernel.org>" into #1 and #3 patches.
- Merge Rob's review comments to the #2 patch.

Main changes v4 --> v5:
- Set the AUX_EN always 1b'1, thus it can fix the regression introduced in v4
  series on Marcel's board.
- Use the lower-case letter in the devicetreee refer to Marcel's comments.
_ Since the default value of the deemphasis parameters are zero, only set
  the deemphasis registers when the input paramters are none zero.

Main changes v3 --> v4:
- Update the yaml to fix syntax error, add maxitems and drop description of phy
- Correct the clock name in PHY DT node.
- Squash the EVK board relalted dts changes into one patch, and drop the
  useless dummy clock and gpio suffix in DT nodes.
- Add board specific de-emphasis parameters as DT properties. Thus each board
  can specify its actual de-emphasis values.
- Update the commit log of PHY driver.
- Remove the useless codes from PCIe driver, since they are moved to PHY driver
- After the discussion and verification of the CLKREQ# configurations with Tim,
  agree to add an optional boolean property "fsl,clkreq-unsupported", indicates
  the CLKREQ# signal is hooked or not in HW designs.
- Add "Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>" tag, since
  Marcel help to test the v3 patch-set.

Main changes v2 --> v3:
- Regarding Lucas' comments.
 - to have a whole view to review the patches, send out the i.MX8MM PCIe support too.
 - move the PHY related bits manipulations of the GPR/SRC to standalone PHY driver.
 - split the dts changes to SOC and board DT, and use the enum instead of raw value.
 - update the license of the dt-binding header file.

Changes v1 --> v2:
- Update the license of the dt-binding header file to make the license
  compatible with dts files.
- Fix the dt_binding_check errors.

Documentation/devicetree/bindings/pci/fsl,imx6q-pcie.yaml    |   6 +++
Documentation/devicetree/bindings/phy/fsl,imx8-pcie-phy.yaml |  92 +++++++++++++++++++++++++++++++
arch/arm64/boot/dts/freescale/imx8mm-evk.dtsi                |  55 +++++++++++++++++++
arch/arm64/boot/dts/freescale/imx8mm.dtsi                    |  46 +++++++++++++++-
drivers/pci/controller/dwc/pci-imx6.c                        |  73 ++++++++++++++++++++++---
drivers/phy/freescale/Kconfig                                |   9 ++++
drivers/phy/freescale/Makefile                               |   1 +
drivers/phy/freescale/phy-fsl-imx8m-pcie.c                   | 237 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
include/dt-bindings/phy/phy-imx8-pcie.h                      |  14 +++++
9 files changed, 525 insertions(+), 8 deletions(-)