staging: mt7621-gpio: add COMPILE_TEST

This driver is actually platform-agnostic.  Add COMPILE_TEST for
the compilation test coverage.
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"
-	depends on SOC_MT7620 || SOC_MT7621
+	depends on SOC_MT7620 || SOC_MT7621 || COMPILE_TEST
 	select GPIO_GENERIC
 	select ARCH_REQUIRE_GPIOLIB
 	help