gpio-backlight: Discover driver during boot time

The gpio-backlight driver seems to be missing the
MODULE_DEVICE_TABLE line which is preventing it from
being modprobed during boot time even if the
gpio-backlight device exists.

This seems to be a bug and this patch attempts to
fix that.

Signed-off-by: Arun Bharadwaj <arun@gumstix.com>
Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>