gpiolib/arches: Centralise bolierplate asm/gpio.h

Rather than requiring architectures that use gpiolib but don't have any
need to define anything custom to copy an asm/gpio.h provide a Kconfig
symbol which architectures must select in order to include gpio.h and
for other architectures just provide the trivial implementation directly.

This makes it much easier to do gpiolib updates and is also a step towards
making gpiolib APIs available on every architecture.

For architectures with existing boilerplate code leave a stub header in
place which warns on direct inclusion of asm/gpio.h and includes
linux/gpio.h to catch code that's doing this.  Direct inclusion of
asm/gpio.h has long been deprecated.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jonas Bonn <jonas@southpole.se>
Acked-by: Tony Luck <tony.luck@intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/gpio/Kconfig

@@ -2,6 +2,14 @@
 # GPIO infrastructure and drivers
 #
 
+config ARCH_HAVE_CUSTOM_GPIO_H
+	bool
+	help
+	  Selecting this config option from the architecture Kconfig allows
+	  the architecture to provide a custom asm/gpio.h implementation
+	  overriding the default implementations.  New uses of this are
+	  strongly discouraged.
+
 config ARCH_WANT_OPTIONAL_GPIOLIB
 	bool
 	help