clock-rpm: Support deferred probe for enable_rpm_scaling

enable_rpm_scaling() does not have a return value and thus it was
not possible to do deferred probing in case rpm-smd driver is not
probed yet.

Add a return value to enable_rpm_scaling() and propagate error from
underlying API calls.

Change-Id: Ifa2fe00aba9e460cf30fae1e0153be17ff55a2df
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>