cnss: Add support to synchronize request_firmware() with suspend/resume

Register for PM notifiers to avoid calling into request_firmware() when
suspend/resume is in progress. Expose the APIs so that WLAN host driver
is able to synchronize request_firmware() with suspend/resume.

CRs-fixed: 701407
Change-Id: I458995bb307d97b0915e7a27c22edc16486e344e
Signed-off-by: Yue Ma <yuem@codeaurora.org>