ARM: zynq: add default ps7_init_gpl.c/h for Zed, MicroZed, ZC70x

Due to licensing issues, the files ps7_init.c/h are not able to be
distributed with U-Boot source code.  Recent Xilinx tools also
provide the GPL variants (ps7_init_gpl.c/h), compatible with U-Boot
license.

Prior to this commit, we had to copy ps7_init files into
board/xilinx/zynq/ before the compile.

To be more user-friendly, let's include ps7_init_gpl.c/h for
Zedboard, MicroZed, ZC702, ZC706.

These init code have been taken from the hwplatform_templates
directory of Xilinx SDK 2014.4.

You can still use customized ps7_init_gpl.c/h by enabling
CONFIG_ZYNQ_CUSTOM_INIT.  The recommended directory for storing them
is now board/xilinx/zynq/custom_hw_platform, but board/xilinx/zynq
is still supported for backward compatibility.  The latter emits
a warning message to prompt users to gradually switch to the new
directory.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>