ARM: sunxi: dts: Enable syscon in DTSI

This patch adds the proper nodes in the DTSI files for Allwinner
A20 and A31.

Signed-off-by: Carlo Caione <carlo@caione.org>
ARM: sunxi: Add syscon support in defconfig

Signed-off-by: Carlo Caione <carlo@caione.org>
ARM: sunxi: Add register bit definitions for SRAM mapping syscon

Signed-off-by: Chen-Yu Tsai <wens@csie.org>