staging: vchiq_arm: Add missing memory barrier comments

One of points on the TODO list was to comment the memory barriers.
This patch addresses the missing ones in order to help reviewers.

Link: https://github.com/raspberrypi/linux/pull/5066


Signed-off-by: Phil Elwell <phil@raspberrypi.com>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Link: https://lore.kernel.org/r/20220624163902.6913-1-stefan.wahren@i2se.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>