rtc: zynqmp: Add calibration set and get support

Zynqmp RTC controller has a calibration feature to compensate
time deviation due to input clock inaccuracy.
Set and get calibration API's are used for setting and getting
calibration value from the controller calibration register.

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220626070817.3780977-3-srinivas.neeli@xilinx.com