sunxi: switch the Allwinner T113 SoC to OF_UPSTREAM

sunxi: switch the Allwinner T113 SoC to OF_UPSTREAM

In contrast to some other Allwinner SoCs, there is no difference between
the DTs for the Allwinner T113-s3 SoC (sun20i) between the U-Boot and the
Linux kernel repository.

Remove the old copies of the T113-s3 related .dts and .dtsi files, and
switch the whole SoC (represented by just one board) over to use
OF_UPSTREAM.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>