backlight: bd6107: Convert to use GPIO descriptor

The Rohm BD6107 driver can pass a fixed GPIO line using the old
GPIO API using platform data. As there are no in-tree users of this
platform data since 2013, we can convert this to use a GPIO descriptor
and require any out-of-tree consumers to pass the GPIO using
a machine descriptor table instead.

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