soc: qcom: scm-mpu: Add an API to unlock the kernel text region

The cpaccess driver writes to the kernel text section during normal
operation. Add an API to disable any protection of this region by the
scm-mpu driver.

Protection is not reenabled after cpaccess operations since this is
expected to be an uncommon usecase.

Change-Id: I83caa906b94f4a522a203c1170f1d913681fc704
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>