ARM: dts: sun8i: a83t: Add pingroup for 8-bit eMMC on mmc2

mmc2 can support 8-bit eMMC chips, with a dedicated reset line.

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