ARM: dts: sun9i: Add pinmux setting for mmc0

mmc0 is only available on port F, and is always used with a 4 bit wide
bus for the onboard micro-sd slot.

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>