arm64: msm: add cpu operations

Add msm cpu operations for arm-v8 cpus. Implement
secondary cpu boot ops and a simple wfi based hotplug
call-back. As a part of this change update device tree
documentation for:

1. Arm cortex-a ACC device which provides percpu reg

2. Armv8 cortex-a compatible string in arm/cpus.txt

Change-Id: I2a26b3a30cd6689db791ee850652d96823e9849f
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>