i2c: i2c-gpio: Enhance driver for buses with shared SCL

Some lower end hardware (especially Realtek based switches) are
designed with multiple I2C buses that share a single clock line.
E.g. the D-Link DGS-1250-28X realizes 4 I2C SFP busses with 5 GPIOs.

Enhance the i2c-gpio driver so it can handle such hardware designs.

- Detect shared SCL GPIOs that are used by multiple I2C buses in the
  devicetree by using a "unique identifier". This is basically the
  phandle and all additional cells.

- The first probing instance allocates and requests the shared SCL
  GPIO with an associated rt_mutex. Subsequent instances detect the
  existing entry via the identifier and increment a reference count
  to reuse the descriptor.

- All data transfers are serialized via custom lock_ops that handle
  both the standard adapter bus lock and the shared SCL mutex. This
  ensures mutual exclusion across adapters sharing the clock line.

- This shared SCL detection works only for dts based systems where
  the GPIO node has at least one cell (usually the pin). GPIOs in
  legacy systems without devicetree will be handled individudally
  as before.

This patch was successfully tested on Linksys LGS310C that has two
SFP slots with two GPIO based I2C buses that share a single SCL.
Test environment: OpenWrt snapshot ported to kernel 6.19.14
including CONFIG_GPIO_SHARED=y and CONFIG_GPIO_SHARED_PROXY=y.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Tested-by: Sander Vanheule <sander@svanheule.net>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>
Link: https://lore.kernel.org/r/20260714162915.3018703-3-markus.stockhausen@gmx.de

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