ARM: OMAP: Fix relative includes for debug-devices.h

As discussed on linux-arm-kernel, we want to avoid
relative includes for the arch/arm/*omap* shared code:

http://www.spinics.net/lists/linux-omap/msg80520.html

Let's add plat/debug-devices.h for debug_card_init()
to fix the relative includes.

Note that drivers must not use this header as it will
break build for omap2+ CONFIG_MULTIPLATFORM builds.

Cc: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/plat-omap/debug-devices.c

@@ -16,7 +16,7 @@
 #include <linux/smc91x.h>
 
 #include <mach/hardware.h>
-#include "../mach-omap2/debug-devices.h"
+#include <plat/debug-devices.h>
 
 /* Many OMAP development platforms reuse the same "debug board"; these
  * platforms include H2, H3, H4, and Perseus2.

arch/arm/plat-omap/include/plat/debug-devices.h

@@ -0,0 +1,2 @@
+/* for TI reference platforms sharing the same debug card */
+extern int debug_card_init(u32 addr, unsigned gpio);