backlight: gpio_backlight: Convert to use GPIO descriptor

This driver is predominantly used by device tree systems, all
of which can deal with modern GPIO descriptors. The legacy
GPIO API is only used by one SH board so make the GPIO
descriptor the default way to deal with it.

As an intended side effect we do not need to look around in
the device tree for the inversion flag since the GPIO
descriptors will intrinsically deal with this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>