ARM: davinci: da850-evm: model the backlight GPIO as an actual device

Instead of enabling the panel backlight in a callback defined in board
file using deprecated legacy GPIO API calls, model the line as a GPIO
backlight device.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>