zynq: Use GPLed files for SPL

The latest Xilinx tools generate ps7_init files that are explicitly
available under GPL. Change the makefile to allow drop in of those files
for building the SPL.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>
Reviewed-and-tested-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>