pinctrl: meson-gxl: Add SPI pins for the SPICC controller

The SPICC controller has dedicated SPI pins, this patchs add the pins
definition in the GXL pinctrl driver.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>