backlight: gpio_backlight: Enable ACPI enumeration

ACPI allows to enumerate specific devices by using compatible strings.
Enable that enumeration for GPIO based backlight devices.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>