pinctrl: meson: Correct the driver GPIO declaration

This should use the provided U_BOOT_DRIVER() macro so that the driver gets
added to the appropriate linker list. Fix it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 7c9dcfed ("pinctrl: meson: rework gx pmx function")
Reported-by: Tom Rini <trini@konsulko.com>
Tested-by: Tom Rini <trini@konsulko.com> on libretech-cc
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>