ARM: OMAP: move debug_card_init() function

debug_card_init() function resides in the plat/board.h file.
Move it to a separate header file under plat/ so the board.h file can be
removed.

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