arm64: allwinner: sun50i-a64: Add dt node for the syscon control module

This patch add the dt node for the syscon register present on the
Allwinner A64.

Only two register are present in this syscon and the only one useful is
the one dedicated to EMAC clock.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>