gpio: gpio-dwapb: add optional reset

Some platforms require reset to be released to allow register
access.

Signed-off-by: Alan Tull <atull@kernel.org>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
[Added DT bindings oneliner for standard reset binding]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>