Add bindings for optional rtc clock pin.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Acked-by: Ajay Singh <ajay.kathat@microchip.com>
Link: https://lore.kernel.org/r/1568037993-4646-1-git-send-email-eugen.hristev@microchip.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>