soc: sunxi: sram: Add updated compatible string for A64 system control

The SRAM mapping controls on Allwinner SoCs is located in a block called
"System Controls". This block also has registers for identifying the SoC,
reading the state of an external boot-related pin, and on some newer SoCs,
glue layer controls for the EMAC Ethernet controller.

The A64 variant compatible is renamed to "allwinner,a64-system-control"
to reflect this. The old A64 compatible is deprecated. So far we haven't
seen any actual use of it.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>