dt-bindings: rtc: sun6i-a31-rtc: Loosen the requirements on the clocks

dt-bindings: rtc: sun6i-a31-rtc: Loosen the requirements on the clocks

The commit 778ae812faa1 ("rtc: sun6i: Make external 32k oscillator
optional") loosened the requirement of the clocks property, making it
optional. However, the binding still required it to be present.

Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Fixes: 778ae812faa1 ("rtc: sun6i: Make external 32k oscillator optional")
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Jernej Skrabec <jernej.skrabec@siol.net>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20210114113538.1233933-3-maxime@cerno.tech