gpio: move gpio-mmio-specific fields out of struct gpio_chip

With all users of bgpio_init() converted to using the modernized generic
GPIO chip API, we can now move the gpio-mmio-specific fields out of
struct gpio_chip and into the dedicated struct gpio_generic_chip. To
that end: adjust the gpio-mmio driver to the new layout, update the
docs, etc.

The changes in gpio-mlxbf2.c and gpio-mpc8xxx.c are here and not in their
respective conversion commits because the former passes the address of
the generic chip's lock to the __releases() annotation and we cannot
really hide it while gpio-mpc8xxx.c accesses the shadow registers in a
driver-specific workaround and there's no reason to make them available
in a public API.

Also: drop the relevant task from TODO as it's now done.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20250910-gpio-mmio-gpio-conv-part4-v2-15-f3d1a4c57124@linaro.org
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

Author: Bartosz Golaszewski

drivers/gpio/TODO

@@ -131,11 +131,6 @@ Work items:
   helpers (x86 inb()/outb()) and convert port-mapped I/O drivers to use
   this with dry-coding and sending to maintainers to test
 
-- Move the MMIO GPIO specific fields out of struct gpio_chip into a
-  dedicated structure. Currently every GPIO chip has them if gpio-mmio is
-  enabled in Kconfig even if it itself doesn't register with the helper
-  library.
-
 -------------------------------------------------------------------------------
 
 Generic regmap GPIO

drivers/gpio/gpio-mlxbf2.c

@@ -156,7 +156,7 @@ static int mlxbf2_gpio_lock_acquire(struct mlxbf2_gpio_context *gs)
  * Release the YU arm_gpio_lock after changing the direction mode.
  */
 static void mlxbf2_gpio_lock_release(struct mlxbf2_gpio_context *gs)
-	__releases(&gs->chip.gc.bgpio_lock)
+	__releases(&gs->chip.lock)
 	__releases(yu_arm_gpio_lock_param.lock)
 {
 	writel(YU_ARM_GPIO_LOCK_RELEASE, yu_arm_gpio_lock_param.io);