arm64: zynqmp: rtc: Update rtc calibration value

As per the design specification
"The 16-bit Seconds Calibration Value represents the number of
 Oscillator Ticks that are required to measure the largest time
 period that is less than or equal to 1 second.
 For an oscillator that is 32.768 KHz, this value will be 0x7FFF."

Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>