wcnss: add API to check if wcnss is undergoing an SSR

Wlan driver should not read registers from wcnss or
try to communicate with the chip when it is undergoing
a subsystem-restart. Added an API with the wcnss platform
driver to check if ssr is in progress, this API will be
used by the wlan driver to protect register access and
other prohibited operations.

Change-Id: Ia0d93e774af126d84dbaabf5c36a2c90fdbd58be
CRs-Fixed: 621974
Signed-off-by: Mihir Shete <smihir@codeaurora.org>