arm64: dts: salvator-x: Add panel backlight support

The panel backlight is controlled through a GPIO and a PWM channel.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
[simon: apply to salvator-x.dtsi instead of r8a7795-salvator-x.dts]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>