staging: mt7621-gpio: update Kconfig with SoC details

Kconfig is using a generic 'Mediatek GPIO Support' in
description and help which is not specific at all about
the current SoC which is MT7621. Update it.

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