wcnss: add driver API to support Thermal Mitigation

The wcnss driver currently supports a sysfs-based mechanism which
allows the Thermal Mitigation function in userspace to read and write
the current thermal mitigation level.  An API is added to allow other
drivers (specifically the WLAN Driver) to register a notifier to be
invoked when the thermal mitigation level is changed.

CRs-Fixed: 317376
Change-Id: Ieda320b618a0a08186db791168e586a86b54d173
Signed-off-by: Jeff Johnson <jjohnson@codeaurora.org>