mailbox: pl320: remove __raw IO

The use of __raw IO accesors is not endian safe and should be used
sparingly. The relaxed variants should be as lightweight and also
are endian safe.

Note, this has not been run-time tested.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>