md: support for asynchronous execution of RAID6 operations

 The following patch-set includes enhancements to the async_tx api and
modifications to md-raid6 to issue memory copies and parity calculations
asynchronously. Thus we may process copy operations and RAID-6 calculations
on the dedicated DMA engines accessible with ASYNC_TX API, and, as a result
off-load CPU, and improve the performance.

 To reduce the code duplication in the raid driver this patch-set modifies
some raid-5 functions to make them possible to use in the raid-6 case.

 The patch-set can be broken down into thee following main categories: 

1) Additions to ASYNC_TX API (patches 1-3)
2) RAID-6 implementation (patches 4-10)
3) ppc440spe ADMA driver (patch 11) (it still has a number of problems,
provided only as a reference here)