arm64: dts: allwinner: h6: Add support for RTC and fix the clock tree

This patch adds RTC node and fixes the clock properties and nodes
to reflect the real clock tree.

The device nodes for the internal oscillator and osc32k are removed,
as these clocks are now provided by the RTC device. Clock references
are fixed accordingly, too.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>