mhi: core: allocate SBL image buffer using CMA pool if configured

Depending on memory configuration, MHI may have a dedicated CMA
pool for buffer management. Always use DMA allocation methods,
so if a memory pool is tied to device, it will allocate memory
from that pool.

CRs-Fixed: 2280387
Change-Id: I60ce09ccf0f04f170461bcea268f963ba8244d29
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>