ARM: dts: sun8i: Add pinmux setting for 8bit mmc2

mmc2 is mostly used with eMMC flash chips, as an alternative to
raw NAND flash chips. 8 bit mmc is commonly used.

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