Add the platform registration for the ep93xx backlight driver that
is now merged.

Since the pwm BRIGHT signal only functions if the video system is
operating, register the backlight when the framebuffer device is
registered.
Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Acked-by: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>