ArmPkg/ArmPsciMpServicesDxe: Move CPU_OFF into a helper function

ApExceptionHandler() and ApProcedure() both end with the same PSCI
CPU_OFF sequence. Move it into a new ApCpuOff() function.

No functional change.

Signed-off-by: Vishal Oliyil Kunnil <vishalo@qti.qualcomm.com>