slim: msm: Control interface clock if it's specified

Some platforms (e.g. 8974) have interface clock that is not ON by
default. Turn it ON during probe, and control it like core clock
if it needs to be controlled.

Change-Id: Ifffb4b7ea9afb5f3f1208b4b2b6638594ffca541
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>