msm: acpuclock-krait: Keep the secondary MUX input fixed

With use of the QSB clock source recently eliminated, only one
input of the secondary clock MUX is used on any target. Since
there is overhead involved reprogramming this MUX when changing
the CPU and L2 frequencies, change the code to just program the
MUX at boot.  Most noticeably, this removes a 1us delay from
every CPU and L2 frequency switch.

Change-Id: I5cd8c981f1be49be7dbba1310d84df439e8be83b
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>
Signed-off-by: Stratos Karafotis <stratosk@semaphore.gr>