shmbridge: some improvements for shmbridge driver

Make change to create and keep memory pool even shmbridge
feature is not supported on certain target, then client can
still suballocte memory from the default bridge; add an API
to query if a bridge exists; check scm_cal return if bridge
feature is supported or not.

Change-Id: Ibe8916acb0d4efa37d4764238accfdd7d370de7f
Signed-off-by: Zhen Kong <zkong@codeaurora.org>