gpio: Get rid of legacy header

A bunch of core gpiolib files still include the <linux/gpio.h>
legacy API header for no good reason. After this only the
gpiolib-legacy.c file includes it, which is fine.

The sysfs ABI code has a pointless wrapper function around
gpio_to_desc() we can just loose.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>