serial: msm-geni-serial: Add support for thaw callback

Hibernation may fail after this driver's freeze
callback.
Implement thaw callback to undo the changes
done in freeze callback to keep system intact.

Change-Id: Ic682e6d4372d4e9da8da4f4b735cef25e152d159
Signed-off-by: Atul Raut <araut@codeaurora.org>