Provide a fraemework for RISC-V ISA extensions

This series implements a generic framework to parse multi-letter ISA
extensions. This series is based on Tsukasa's v3 isa extension improvement
series[1]. I have fixed few bugs and improved comments from that series
(PATCH1-3). I have not used PATCH 4 from that series as we are not using
ISA extension versioning as of now. We can add that later if required.

PATCH 4 allows the probing of multi-letter extensions via a macro.
It continues to use the common isa extensions between all the harts.
Thus hetergenous hart systems will only see the common ISA extensions.

PATCH 6 improves the /proc/cpuinfo interface for the available ISA extensions
via /proc/cpuinfo.

Here is the example output of /proc/cpuinfo:
(with debug patches in Qemu and Linux kernel)

/ # cat /proc/cpuinfo
processor	: 0
hart		: 0
isa		: rv64imafdcsu
isa-ext		: sstc,sscofpmf
mmu		: sv48

processor	: 1
hart		: 1
isa		: rv64imafdcsu
isa-ext		: sstc,sscofpmf
mmu		: sv48

processor	: 2
hart		: 2
isa		: rv64imafdcsu
isa-ext		: sstc,sscofpmf
mmu		: sv48

processor	: 3
hart		: 3
isa		: rv64imafdcsu
isa-ext		: sstc,sscofpmf
mmu		: sv48

Anybody adding support for any new multi-letter extensions should add an
entry to the riscv_isa_ext_id and the isa extension array. 
E.g. The patch[2] adds the support for sstc extension.

[1] https://lore.kernel.org/all/0f568515-a05e-8204-aae3-03597...
[2] https://github.com/atishp04/linux/commit/dfc9b0d16f5a6e46... 


Changes from v1->v2:
1. Instead of adding a separate DT property use the riscv,isa property.
2. Based on Tsukasa's v3 isa extension improvement series.

Atish Patra (3):
RISC-V: Implement multi-letter ISA extension probing framework
RISC-V: Do no continue isa string parsing without correct XLEN
RISC-V: Improve /proc/cpuinfo output for ISA extensions

Tsukasa OI (3):
RISC-V: Correctly print supported extensions
RISC-V: Minimal parser for "riscv, isa" strings
RISC-V: Extract multi-letter extension names from "riscv,isa"

arch/riscv/include/asm/hwcap.h |  25 +++++++
arch/riscv/kernel/cpu.c        |  44 ++++++++++-
arch/riscv/kernel/cpufeature.c | 132 ++++++++++++++++++++++++++++-----
3 files changed, 180 insertions(+), 21 deletions(-)

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2.30.2