backlight: pwm_bl: Switch to full GPIO descriptor

The PWM backlight still supports passing a enable GPIO line as
platform data using the legacy <linux/gpio.h> API.

It turns out that ever board using this mechanism except one
is pass .enable_gpio = -1. So we drop all these cargo-culted -1's
from all instances of this platform data in the kernel.

The remaning board, Palm TC, is converted to pass a machine
descriptior table with the "enable" GPIO instead, and delete the
platform data entry for enable_gpio and the code handling it
and things should work smoothly with the new API.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>