basic_mmio_gpio: detect output method at probe time

Rather than detecting the output method each time in the .set()
callback, do it at probe time and set the appropriate callback.

Changes since v2: moved the reg_dat initialization into
bgpio_setup_io().

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