uio: msm_sharedmem: Add custom mmap

Uio's default mmap for physical memory uses
pgprot_noncached which is strongly ordered.

Using pgprot_writecombine would allow for
unalligned access.

Add a custom mmap function when registering uio
to allow unalligned access to the mmaped
region.

Change-Id: If314eade4913bafd53d2b4abc6b205385e29dfd3
Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>