ARM: OMAP: resolve sparse warning concerning debug_card_init()

Commit 801475cc ("ARM: OMAP: move
debug_card_init() function") results in the following new sparse
warning:

arch/arm/plat-omap/debug-devices.c:71:12: warning: symbol 'debug_card_init' was not declared. Should it be static?

Fix by implementing Tony's suggestion to add a "sideways include" to the
new location of the debug-devices.h file in arch/arm/mach-omap2/.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>