ARM: dts: sun8i: r40: Add RTC device node

The R40 has an RTC hardware block, which has additional registers
that are not related to RTC or clock functions, and is otherwise
compatible with the H3's RTC.

Add a device node for it, and fix up any references to the LOSC.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Tested-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>