arm64: allwinner: a64: Add RTC clock to phandle 32kHz external oscillator

Outside of SOC few chips need external clock source
through RTC example Wifi chip. So RTC clock nodes to
phandle 32kHz external oscillator.

prefix rtc- with clock-output-names defined in
dt-binding to avoid confusion with existing osc32k name.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>