gpio: rename gpio-generic.c into gpio-mmio.c

This patch renames the gpio-generic.c into gpio-mmio.c.
This is because currently the file only contains code
for a memory-mapped GPIO driver. There isn't any support
for ioports or other resource type.

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>