ARM: OMAP: rx51: Register platform device for rx51_battery

This is needed to make the battery monitor actually work on Nokia N900.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>