microblaze: Fix dma alloc and free coherent dma functions

We have to use consistent code to be able to do coherent dma
function. In consistent code is used cache inhibit page mapping.
Xilinx reported that there is bug in Microblaze for WB and
d-cache_always use option. Microblaze 7.30.a should be first version
where is this bug removed.

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