arm: mach-omap2: rx51: clean up dummy onenand init

When OneNAND support is disabled, the platform code defines NULL
board_onenand_data and empty init function for us. By utilizing this we
can avoid cluttering board files with dummy definitions/wrappers.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>