ARM: OMAP: Remove debug-devices.c

The OMAP2420 H4 board was the only board using the
plat-omap/debug-devices.c code for configuring ethernet support. Now
that OMAP2420 H4 has been migrated to use the generic gpmc code for
configuring ethernet support, the debug-devices.c file is no longer
used and so remove it and its header file.

Signed-off-by: Jon Hunter <jon-hunter@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>