msm: clock-rpm: Move enable rpm scaling call into rpm clock driver

Different targets may share the enable_rpm_scaling function to enable
the rpm clock scaling feature on RPM. Therefore, move this function
to the common rpm clock driver so that different clock drivers can
use it.

Change-Id: Ice59d0ac4f43b42e4ab3d2f9e564c28dff42e275
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>