ARM: shmobile: armadillo: Set backlight enable GPIO

The Armadillo 800 EVA panel module has a backlight enable signal
connected to GPIO 61. Instead of requesting the GPIO in board code and
setting it to a high level unconditionally, pass the GPIO number to the
PWM backlight driver as the backlight enable GPIO.

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