basic_mmio_gpio: allow overriding number of gpio

Some platforms may have a number of GPIO that is less than the register
width of the peripheral.

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