zynq: treat ps7_init.c/h as external files to ignore them

zynq: treat ps7_init.c/h as external files to ignore them

ps7_init.c and ps7_init.h are supposed to be exported by hw project
and copied to board/xilinx/zynq/ directory.

We want them to be ignored by git.
So what we should do is to always treat them as external files
rather than replacing ps7_init.c

This commit does:

 - Move a weak function ps7_init() to arch/arm/cpu/armv7/zynq/spl.c
   and delete board/xilinx/zynq/ps7_init.c

 - Compile board/xilinx/zynq/ps7_init.c only when it exists

 - Add .gitignore to ignore ps7_init.c/h

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