firmware: arm_scpi: fix incorrect __iomem accesses using correct accessors

At several positions in the code sparse complains about incorrect access
to __iomem annotated memory. Fix this and make sparse happy.

Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
[Sudeep Holla: changed the patch title to describe the change]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>