zynq: add empty xil_io.h to avoid compile error

zynq: add empty xil_io.h to avoid compile error

ps7_init.c exported by hw project has #include "xil_io.h" line
but U-Boot does not have "xil_io.h".

So we get an error on SPL build:
  ps7_init.c:12581:20: fatal error: xil_io.h: No such file or directory

We can delete the include directive in ps7_init.c to avoid this error.
But it is painful to do this every time we export ps7_init.c file.

Instead, we can put an empty xil_io.h in the same directory
so we can directly copy ps7_init.c as is.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Tom Rini <trini@ti.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>