soc: zynqmp: Make zynqmp_pm_late_init() static

It removes sparse warning:
drivers/soc/xilinx/zynqmp/sleep.c:44:12: warning: symbol
'zynqmp_pm_late_init' was not declared. Should it be static?

Signed-off-by: Michal Simek <michal.simek@xilinx.com>