arcnet: arcdevice.h: Add arcnet_readb and arcnet_writeb

Use the same indirection as the other arcnet_<I/O> macros.
Neither of these new macros add the BUS_ALIGN use for 8 bit devices
on 16 bit busses.

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