ARM: dts: sun6i: sina31s: Switch to mmc3 for onboard eMMC

According to Allwinner, only mmc3 supports 8 bit DDR transfers for eMMC.
Switch to mmc3 for the onboard eMMC, and also assign vqmmc for signal
voltage sensing, and "cap-mmc-hw-reset" to denote this instance can use
eMMC hardware reset.

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