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>

drivers/staging/mt7621-gpio/Kconfig

 config GPIO_MT7621
-	bool "Mediatek GPIO Support"
+	bool "Mediatek MT7621 GPIO Support"
 	depends on SOC_MT7620 || SOC_MT7621 || COMPILE_TEST
 	select GPIO_GENERIC
 	select GPIOLIB_IRQCHIP
 	select ARCH_REQUIRE_GPIOLIB
 	help
-	  Say yes here to support the Mediatek SoC GPIO device
+	  Say yes here to support the Mediatek MT7621 SoC GPIO device