dt-bindings: arm: sunxi: document MangoPi MQ-R board names

dt-bindings: arm: sunxi: document MangoPi MQ-R board names

The MangoPi MQ-R board is a small development board, using Allwinner
SoCs with co-packaged DRAM. There are versions with a RISC-V core and
ones with two Arm Cortex-A7 cores.

Add the board/SoC compatible string pair to the list of known boards.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230320005249.13403-4-andre.przywara@arm.com
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>