gpiolib: Make the legacy <linux/gpio.h> consumer-only

The legacy <linux/gpio.h> header was an all-inclusive header used
by drivers and consumers alike. After eliminating the last users
of the driver defines, we can drop the inclusion of the
<linux/gpio/driver.h> header.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>