ARM: dts: at91: sama5d2: add an rtc label for derived boards

Add an rtc label so we just need to alias it from derived boards.

Signed-off-by: Kamel Bouhara <kamel.bouhara@bootlin.com>
Link: https://lore.kernel.org/r/20191002145914.14874-1-kamel.bouhara@bootlin.com


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