gpio: devm_gpio_* support should not depend on GPIOLIB

Some architectures (e.g. blackfin) provide gpio API without requiring
GPIOLIB support (ARCH_WANT_OPTIONAL_GPIOLIB).  devm_gpio_* functions
should also work for these architectures, since they do not really
depend on GPIOLIB.

Add a new option GPIO_DEVRES (enabled by default) to control the build
of devres.c.  It also removes the empty version of devm_gpio_*
functions for !GENERIC_GPIO build from linux/gpio.h, and moves the
function declarations from asm-generic/gpio.h into linux/gpio.h.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/gpio/Kconfig

@@ -30,6 +30,9 @@ config ARCH_REQUIRE_GPIOLIB
 	  Selecting this from the architecture code will cause the gpiolib
 	  code to always get built in.
 
+config GPIO_DEVRES
+	def_bool y
+	depends on HAS_IOMEM
 
 
 menuconfig GPIOLIB