arm64: dts: allwinner: a64: add SRAM controller device tree node

Allwinner A64 has a SRAM controller, and in the device tree currently
we have a syscon node to enable EMAC driver to access the EMAC clock
register. As SRAM controller driver can now export regmap for this
register, replace the syscon node to the SRAM controller device node,
and let EMAC driver to acquire its EMAC clock regmap.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
[wens@csie.org: Updated compatible string]
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>