rtc: sun6i: Make external 32k oscillator optional

rtc: sun6i: Make external 32k oscillator optional

Some boards, like OrangePi PC2 (H5), OrangePi Plus 2E (H3) and Tanix TX6
(H6) don't have external 32kHz oscillator. Till H6, it didn't really
matter if external oscillator was enabled because HW detected error and
fall back to internal one. H6 has same functionality but it's the first
SoC which have "auto switch bypass" bit documented and always enabled in
driver. This prevents RTC to work correctly if external crystal is not
present on board. There are other side effects - all peripherals which
depends on this clock also don't work (HDMI CEC for example).

Make clocks property optional. If it is present, select external
oscillator. If not, stay on internal.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20200308135849.106333-2-jernej.skrabec@siol.net
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>