gpio: shared: handle the reset-gpios corner case

gpio: shared: handle the reset-gpios corner case

There's an unexpected interaction between the reset-gpio driver and the
shared GPIO support. The reset-gpio device is an auxiliary device that's
created dynamically and fulfills a similar role to the gpio-shared-proxy
driver but is limited in scope to just supporting the "reset-gpios"
property.

The shared GPIO core code does not take into account that the machine
lookup entry we create when scanning the device-tree must connect the
reset-gpio device - that is the actual consumer of the GPIO and not the
consumer defined on the device tree, which in turn consumes the shared
reset control exposed by the reset-gpio device - to the GPIO controller.

We also must not skip the gpio-shared-proxy driver as it's possible that
a shared GPIO may be used by one consumer as a reset-gpios going through
the reset-gpio device and another that uses GPIOLIB.

We need to make it a special case handled in gpiolib-shared.c. Add a new
function - gpio_shared_dev_is_reset_gpio() - whose role it is to verify
if a non-matching consumer of a shared pin is a reset-gpio device and
make sure it's the right one for this pin. To that end make sure that
its parent is the GPIO controller in question and that the fwnode we
identified as sharing the pin references that controller via the
"reset-gpios" property.

Only include that code if the reset-gpio driver is enabled.

Fixes: a060b8c511ab ("gpiolib: implement low-level, shared GPIO support")
Reported-by: Val Packett <val@packett.cool>
Closes: https://lore.kernel.org/all/3b5d9df5-934d-4591-8827-6c9573a6f7ba@packett.cool/
Tested-by: Val Packett <val@packett.cool>
Tested-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20251125-gpiolib-shared-reset-gpio-fix-v2-1-4eb6fa41f1dd@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>