rtc: zynqmp: re-use rtc_time64_to_tm operation

This allows a subsequent commit to spin_unlock sooner.

Signed-off-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/20191128015613.10003-1-jeff.dagenais@gmail.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>