dt-bindings: arm: sunxi: Add compatible for A64 OlinuXino with eMMC

A64 OLinuXino board from Olimex has three variants with onboard eMMC:
A64-OLinuXino-1Ge16GW, A64-OLinuXino-1Ge4GW and A64-OLinuXino-2Ge8G-IND. In
addition, there are two variants without eMMC. One without eMMC and one with SPI
flash. This suggests the need for separate device tree for the three eMMC
variants.

Add new compatible string to the bindings documentation for the A64 OlinuXino
board variant with on-board eMMC.

Signed-off-by: Sunil Mohan Adapa <sunil@medhas.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>