gpiolib: implement low-level, shared GPIO support

This module scans the device tree (for now only OF nodes are supported
but care is taken to make other fwnode implementations easy to
integrate) and determines which GPIO lines are shared by multiple users.
It stores that information in memory. When the GPIO chip exposing shared
lines is registered, the shared GPIO descriptors it exposes are marked
as shared and virtual "proxy" devices that mediate access to the shared
lines are created. When a consumer of a shared GPIO looks it up, its
fwnode lookup is redirected to a just-in-time machine lookup that points
to this proxy device.

This code can be compiled out on platforms which don't use shared GPIOs.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20251112-gpio-shared-v4-3-b51f97b1abd8@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

drivers/gpio/Kconfig

@@ -6,6 +6,9 @@
 config GPIOLIB_LEGACY
 	def_bool y
 
+config HAVE_SHARED_GPIOS
+	bool
+
 menuconfig GPIOLIB
 	bool "GPIO Support"
 	help
@@ -50,6 +53,11 @@ config OF_GPIO_MM_GPIOCHIP
 	  this symbol, but new drivers should use the generic gpio-regmap
 	  infrastructure instead.
 
+config GPIO_SHARED
+	def_bool y
+	depends on HAVE_SHARED_GPIOS || COMPILE_TEST
+	select AUXILIARY_BUS
+
 config DEBUG_GPIO
 	bool "Debug GPIO calls"
 	depends on DEBUG_KERNEL