rtc: sun6i: Add support for the external oscillator gate

The RTC can output its 32kHz clock outside of the SoC, for example to clock
a WiFi chip.

Create a new clock that other devices will be able to retrieve, while
maintaining the DT stability by providing a default name for that clock if
clock-output-names doesn't list one.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>