devicetree: bindings: rtc: add bindings for xilinx zynqmp rtc

adds file for description on device node bindings for RTC
found on Xilinx Zynq Ultrascale+ MPSoC.

Signed-off-by: Suneel Garapati <suneel.garapati@xilinx.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>