soc: qcom: smem: Add probe deferral support

SMEM APIs may be called before probe() has a had a chance to initialize
the driver.  In such cases, EPROBE_DEFER should be returned to the caller
to indicate SMEM needs more time to probe.

Change the public APIs to detect the current status of probe() and return
EPROBE_DEFER if necessary.

Change-Id: Idebeeccc48b5fe02a219eee746d37b264047a08a
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>