sunxi: A64: Re-add syscon to DT node

The sun50i-a64.dtsi changes introduced in Linux v4.19-rc1 changed the
compatible name for the syscon controller, dropping the generic "syscon"
fallback. Using this new DT node will make the Ethernet driver in every
older kernel (or non-Linux kernels) fail to initialise the MAC device.

To allow booting distribution kernels (from installer images via UEFI,
for instance), re-add the syscon compatible string as a fallback. This
works with both older and newer kernels.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>