leds: gpio: Try to lookup gpiod from device

This augments the GPIO lookup code in the GPIO LEDs to
attempt to look up a GPIO descriptor from the device with
index.

This makes it possible to use GPIO machine look-up tables
and stop passing global GPIO numbers through platform data.

Using this we can stepwise convert existing board files
to use machine descriptor tables and then eventually drop
the legacy GPIO support and only include
<linux/gpio/consumer.h> and use descriptors exclusively.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>