ARM: shmobile: armadillo: dts: Add PWM backlight enable GPIO

The Armadillo 800 EVA panel module has a backlight enable signal
connected to GPIO 61. Report this in the backlight DT node.

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