arcnet: Add arcnet_<I/O> macros

arcnet uses an I/O scheme which can align I/O addresses to word boundaries
on different architectures.

Add arcnet specific macros which can hide this alignment calculation.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>