uio: msm_sharedmem: Add a function to release the shared RAM section

A function to free the shared RAM section has been added to the
msm_sharedmem driver. This function is needed to free the RAM section
on shutdown of the msm_sharedmem driver. The function is called on
the shutdown command for the driver.

Change-Id: Iff04f93d380664c5e3df9b0cd575cf9e6300ea39
Signed-off-by: Anant Goel <anantg@codeaurora.org>