realtek: add new i2c-gpio-shared driver for shared SCL lines

realtek: add new i2c-gpio-shared driver for shared SCL lines

Some Realtek switches have been designed with I2C busses that share a
single SCL line. The clock line is used for 2 or more busses. This cannot
be used with the standard i2c-gpio driver that relies on distinct SDA
and SCL pairs.

Provide a derived i2c-gpio-shared driver that can be used instead. This
driver can handle up to 4 busses with only a single clock line.

Signed-off-by: Markus Stockhausen <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18737
Signed-off-by: Robert Marko <[email protected]>