msm: ramdump: Add destroy_ramdump_device() function

If an SSR driver is detached there is no way to free the ramdump
device that is allocated in create_ramdump_device(). Add a
destroy_ramdump_device() function to clean up memory. Admittedly
this doesn't try to solve the race condition in destroying the
device while do_ramdump() is in progress, but it is a good first
start.

Change-Id: Ib46ac8589a9ae7405441328eb9bfa0353362513a
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>