staging: mt7621-gpio: avoid use banks in device tree

Banks shouldn't be defined in DT if number of resources
per bank is not variable. We actually know that this SoC
has three banks so take that into account in order to don't
overspecify the device tree. Device tree will only have one
node making it simple. Update device tree, binding doc and
code accordly.

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