arm64: zynqmp: xil_io.h: declare functions as static

Fixes sparse warnings when building zynqmp defconfigs:
  ./board/xilinx/zynqmp/xil_io.h:12:6: warning: symbol 'Xil_Out32' was not declared. Should it be static?
  ./board/xilinx/zynqmp/xil_io.h:17:5: warning: symbol 'Xil_In32' was not declared. Should it be static?
  ./board/xilinx/zynqmp/xil_io.h:22:6: warning: symbol 'usleep' was not declared. Should it be static?

Also add __maybe_unused to usleep() since it is not used by minimized
psu_init_gpl.c files, so it would warn as "defined but not used".

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>