hwrng: Get clock source based on dtsi flag

On few devices prng module is placed inside hardware
KM block. Add changes to get clock node based on the
dtsi flag to differentiate between different clock
sources.

Change-Id: Ib6b7605606a0c666b33935f2ca28703a83b5b778
Signed-off-by: AnilKumar Chimata <anilc@codeaurora.org>