ARM: dts: sun9i: Add 8 bit mmc pinmux setting for mmc2

mmc2 is available on port C. Add a pinmux setting for 8 bit wide eMMC.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>