basic_mmio_gpio: convert to non-__raw* accessors

The __raw_* accessors don't include memory barriers and can cause
problems when writes get stuck in write buffers.

Suggested-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>