sunxi: add MangoPi MQ-R board support

The MangoPi MQ-R board uses an Allwinner T113s Soc (with 128MB of
embedded DRAM), support for which was just added to the code.

Since the devicetree was already synced from the latest Linux kernel
tree, all we need is a _defconfig file to add support for the board.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>