rtc: add xilinx zynqmp rtc driver

Add support for RTC controller found on Xilinx Zynq Ultrascale+ MPSoC
platform.

Signed-off-by: Suneel Garapati <suneel.garapati@xilinx.com>
Acked-by: Moritz Fischer <moritz.fischer@ettus.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>