dt-bindings: rtc: sun6i-rtc: Deprecate external clock output for A31

The A31 does not have an external clock output directly from the RTC.
Instead, it has four muxable clock outputs: three (A, B, C) are
controlled from the CCU, and the last (D) is controlled from the PRCM.

Deprecate the usage of the external clock output for the A31 compatible.

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>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>