arm64: zynqmp: Add empty sleep.h file for psu_init* compilation

psu_init* contain sleep.h header which is not present in u-boot.
Instead of keep comment sleep.h in psu_init* it is easier to add empty
file which is included.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/board/xilinx/zynqmp/sleep.h b/board/xilinx/zynqmp/sleep.h
new file mode 100644
index 0000000..a962319
--- /dev/null
+++ b/board/xilinx/zynqmp/sleep.h
@@ -0,0 +1 @@
+/* Intentionally empty file for psu_init* */