memshare: Free the memory after XPU unlocking is done

Currently memshare driver is listening to notification
SUBSYS_BEFORE_POWERUP, which doesn't guarantee the unlocking
of XPU for the clients sitting on peripheral in case for watchdog
bite. This could lead to a XPU violation. Move the notification
to free up memory during SUBSYS_AFTER_POWERUP to ensure that the
allocated memory XPU has been removed.

Change-Id: Ib19c9172bc282381ab372d197d927a7842298110
Signed-off-by: Katish Paran <kparan@codeaurora.org>