omap: Fix matrix_keymap_data usage

Otherwise we'll get compile errors like:

arch/arm/plat-omap/include/mach/keypad.h:38:1: warning: "KEY" redefined
arch/arm/plat-omap/include/mach/keypad.h:39:1: warning: "PERSISTENT_KEY" redefined
...

Signed-off-by: Tony Lindgren <tony@atomide.com>