dt-bindings: rtc: add device-tree bindings for the Amlogic Meson RTC

The 32-bit Amlogic Meson SoCs (Meson6, Meson8, Meson8b and Meson8m2)
have a built-in RTC block.
It has the following inputs:
- an 32.768kHz crystal oscillator
- an interrupt line
- a reset line
- 0.9V voltage input

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
[resurrected patches from Ben after 2 years]
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>