mhi: core: Add support to pre allocate image buffers

This allows controller to keep the memory allocated for bhi
vector table for rddm and fbc images. Memory remains allocated
after controller powers down. Add a controller flag to make the
decision for memory allocation.

Change-Id: Ic8aee54ae57f5f9351158b18cd96faf7eeb81429
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>