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>

include/linux/gpio/consumer.h

@@ -2,9 +2,10 @@
 #ifndef __LINUX_GPIO_CONSUMER_H
 #define __LINUX_GPIO_CONSUMER_H
 
+#include <linux/bits.h>
 #include <linux/bug.h>
+#include <linux/compiler_types.h>
 #include <linux/err.h>
-#include <linux/kernel.h>
 
 struct device;
 
@@ -189,6 +190,8 @@ struct gpio_desc *devm_fwnode_gpiod_get_index(struct device *dev,
 
 #else /* CONFIG_GPIOLIB */
 
+#include <linux/kernel.h>
+
 static inline int gpiod_count(struct device *dev, const char *con_id)
 {
 	return 0;