staging: mt7621-gpio: avoid use of globals and use platform_data instead

Gpio driver have a some globals which can be avoided just
using platform_data in a proper form. This commit adds a new
struct mtk_data which includes all of those globals setting them
using platform_set_drvdata and devm_gpiochip_add_data functions.
With this properly set we are able to retrieve driver data along
the code using kernel api's so globals are not needed anymore.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>