This commit fixes potential unpredictable behavior, caused by using
the ^ form of ldmia instruction, while exiting an exception in SMP
mode on Cortex-A/R.
Change:
Use "pop" instead of "ldmia" to restore user mode registers while
exiting from an exception via `z_arm_cortex_ar_exit_exc`.
Reason for change:
Processor mode is always set to system (MODE_SYS) before calling
`z_arm_cortex_ar_exit_exc` and hence, the user mode register can be
accessed directly without the ^ form of the instruction. Also, LDMIA
instruction is UNPREDICTABLE in SYStem mode.
Signed-off-by: Sudan Landge <sudan.landge@arm.com>