arm64: dts: salvator-common: add 12V regulator to backlight

This fixes the warning "pwm-backlight backlight: backlight supply power
not found, using dummy regulator".

Fixes: b33be336 ("arm64: dts: salvator-x: Add panel backlight support")
Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>