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>