hw/i2c: flatten pca954x mux device

hw/i2c: flatten pca954x mux device

Previously this device created N subdevices which each owned an i2c bus.
Now this device simply owns the N i2c busses directly.

Tested: Verified devices behind mux are still accessible via qmp and i2c
from within an arm32 SoC.

Reviewed-by: Hao Wu <wuhaotsh@google.com>
Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20220202164533.1283668-1-venture@google.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>