Keyboard: omap-keypad: use matrix_keypad.h

Most keypad drivers make use of the <linux/input/matrix_keypad.h>
defined macros, structures and inline functions.

Convert omap-keypad driver to use those as well, as suggested by a
compile time warning, hardcoded into the OMAP <palt/keypad.h>.

Created against linux-2.6.37-rc5.
Tested on Amstrad Delta.
Compile tested with omap1_defconfig and omap2plus_defconfig shrinked to
board-h4.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Reviewed-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Tony Lindgren <tony@atomide.com>