msm: clock-rpm: Remove support for older RPM communication method

All RPMs now support communication via SMD. clock-rpm no longer
needs to use the older register based communication mechanism.
Remove support for it.

Change-Id: I547fb942897711f0c186855207bee92defe62dfd
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>