gpio: Avoid kernel.h inclusion where it's possible

gpio: Avoid kernel.h inclusion where it's possible

Inclusion of kernel.h increases the mess with the header dependencies.
Avoid kernel.h inclusion where it's possible.

Besides that, clean up a bit other inclusions inside GPIO subsystem headers.
It includes:
 - removal pin control bits (forward declaration and header) from linux/gpio.h
 - removal of.h from asm-generic/gpio.h
 - use of explicit headers in gpio/consumer.h
 - add FIXME note with regard to gpio.h inclusion in of_gpio,h

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200205134336.20197-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>