drivers: gpio: mt7621: use devm_platform_ioremap_resource()

Use the new helper that wraps the calls to platform_get_resource()
and devm_ioremap_resource() together.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>