msm: adsprpc: Option to keep maps persistent

ATTR_KEEP_MAP should allow the maps to be persistent and
unmap happens during file close or explicit unmap request.

Change-Id: I16b8738be21dcc9f9ae0b1e87e37810c3829bde3
Acked-by: Bharath Kumar <bkumar@qti.qualcomm.com>
Signed-off-by: Tharun Kumar Merugu <mtharu@codeaurora.org>