microblaze: Support for WB cache

Microblaze version 7.20.d is the first MB version which can be run
on MMU linux. Please do not used previous version because they contain
HW bug.
Based on WB support was necessary to redesign whole cache design.
Microblaze versions from 7.20.a don't need to disable IRQ and cache
before working with them that's why there are special structures for it.

Signed-off-by: Michal Simek <monstr@monstr.eu>