basic_mmio_gpio: support direction registers

Most controllers require the direction of a GPIO to be set by writing to
a direction register.  Add support for either an input direction
register or an output direction register.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>