msm: clock-local: Make branch ctl_reg optional

Branch clocks will soon be used for reset-only purposes, in which
case the ctl_reg field will be NULL. Update the branch handoff
and disable code to check for a NULL register address before
reading the register.

Change-Id: I728b2c1ab2ea6a25010b15b3fb553390003175e2
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>