dt-bindings: rtc: sun6i: Add compatible for H6 RTC

RTC on H6 is similar to the one on H5 SoC, but incompatible in small
details. See the driver for description of differences. For example
H6 RTC needs to enable the external low speed oscillator. Add new
compatible for this RTC.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Link: https://lore.kernel.org/r/20190820151934.3860-2-megous@megous.com


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