ARM: zynq: Add default ps7_init_gpl.c/h for ZYBO

Add ps7_init_gpl.c/h for the ZYBO board. This instance of the ps7_init
is generated by the Vivado 2015.3 tools using the system configuration
provided by Digilent located on their website.

Update the kconfig so that the defconfig is not overrided to use the
custom init ps7_init_gpl target by default.

Signed-off-by: Nathan Rossi <nathan@nathanrossi.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: "Sören Brinkmann" <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>