ARM: dts: sun6i: Add mmc2_pins_a pinmux setting to sun6i-a31.dtsi

Add a pinmux setting for using mmc2 in regular 4 bit mode.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>