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>
diff --git a/arch/arm/cpu/armv7/zynq/spl.c b/arch/arm/cpu/armv7/zynq/spl.c
index c3ea221..d73e5cb 100644
--- a/arch/arm/cpu/armv7/zynq/spl.c
+++ b/arch/arm/cpu/armv7/zynq/spl.c
@@ -74,3 +74,11 @@
 	return 0;
 }
 #endif
+
+__weak void ps7_init(void)
+{
+	/*
+	 * This function is overridden by the one in
+	 * board/xilinx/zynq/ps7_init.c, if it exists.
+	 */
+}