ARM: dts: sunxi: Use constants for RTC clock indexes

The binding header provides descriptive names for the RTC clock indexes,
since the indexes were arbitrarily chosen by the binding, not by the
hardware. Let's use the names, so the meaning is clearer.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20220607012438.18183-1-samuel@sholland.org