i2c: mux: Allow muxes to work as children of i2c bus without i2c-parent

i2c: mux: Allow muxes to work as children of i2c bus without i2c-parent

For mux check if the parent is already a device of UCLASS_I2C and if yes
just use that. Otherwise see if someone specified an i2c-parent phandle.
This mimics the behavior found in the Kernel, as it removes the
requirement to explicitly specify a i2c-parent phandle.

Signed-off-by: Moritz Fischer <[email protected]>
Cc: Heiko Schocher <[email protected]>
Cc: Bin Meng <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Michal Simek <[email protected]>
Cc: [email protected]