arm64: allwinner: h6: add the basical Allwinner H6 DTSI file

Allwinner H6 is a new SoC with Cortex-A53 cores from Allwinner, with its
memory map fully reworked and some high-speed peripherals (PCIe, USB
3.0) introduced.

This commit adds the basical DTSI file of it, including the clock
support and UART support.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>