msm: pil: Add memory map and unmap function ptrs

Certain platforms require the ability to do custom mapping and
unmapping of firmware  memory regions.  Add map and umap function
pointers to the descriptor, and initialize them to the existing
functions by default if none are specified by the user.

Change-Id: I9ef7cab266e45ba920e7ec20dd281a92909b4f74
Acked-by: Kaushik Sikdar <ksikdar@qti.qualcomm.com>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>