msm_shared: scm: add support for armv8 register width mode switch

The appropriate register width for the kernel to be booted
may not match the current armv8 execution mode. Utilize
exception level change through secure monitor to request
the register width setting during an exception return, instead
of using a regular branch.

Change-Id: I004c12a1889ce53013da4860d9bea13f7597c941
3 files changed