ARM: shmobile: armadillo800eva: Add backlight support

The flat panel backlight on the Armadillo 800 EVA board is driven by the
TPU PWM output.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>