uio: msm_sharedmem: Add stub shutdown function

A stub shutdown function is added as the default
implementation. This function is used by targets
that do not need to free their memory when the
driver is shutdown.

Change-Id: I073cda4fee7a1c6c34c5ba72d9ba73478ef2d90d
Signed-off-by: Anant Goel <anantg@codeaurora.org>