wcnss: Add Riva 'power on lock' APIs

During SSR Riva should not be 'powered on' until all the host
drivers finish their shutdown routines. Add APIs to support
this synchronization mechanism.

Change-Id: Id211cf2deb7accd0e4bdfe8cd328e6807eac875e
Signed-off-by: Sameer Thalappil <sameert@codeaurora.org>