ARM: dts: sun8i: Add eMMC dt node on Orangepi Plus boards

The Orangepi Plus has a 16GB eMMC, the vcc, the lack of pull-ups and
the use of the hw-reset pin have all been verified with the board
schematic.

With this dts node for mmc2, the eMMC runs at the following ios settings:
clock:          52000000 Hz
vdd:            21 (3.3 ~ 3.4 V)
bus mode:       2 (push-pull)
chip select:    0 (don't care)
power mode:     2 (on)
bus width:      3 (8 bits)
timing spec:    8 (mmc DDR52)
signal voltage: 0 (3.30 V)
driver type:    0 (driver type B)

Note the mmcblk1boot0/boot1 partitions are unused as the BROM will load
the SPL from 8k from the start of the main blockdev, just as with a
regular sdcard in mmc0.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>