msm: rng: Add support for iface clk

Currently the driver supports only enabling core_clk,but on
certain targets, iface_clk is used for the hardware RNG
block. This fix adds compatibility to targets that have iface
clock instead of the core clock.

Change-Id: I480c3c7070e09f945439ea48e6877c7170ceeeb9
Signed-off-by: Hariprasad Dhalinarasimha <hnamgund@codeaurora.org>