dt-bindings: sunxi-sram: add binding for Allwinner H6 SRAM C

The Allwinner H6 SoC's DE3 needs the SRAM C section being claimed in the
system controller to work, like A64 DE2.

As H6 and A64 system controller are quite similar, code is reused now,
and the A64 fallback compatible string is added after the H6 compatible
string.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[fixed typo in compatible string]
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>