rtc: zynqmp: Updated calibration value

As per RTC spec default calibration value is 0x7FFF.
We are in process to update the 0x7FFF as default value in
the next version of TRM.

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220626070817.3780977-2-srinivas.neeli@xilinx.com