hw/i2c: add pca954x i2c-mux switch

hw/i2c: add pca954x i2c-mux switch

The pca954x is an i2c mux, and this adds support for two variants of
this device: the pca9546 and pca9548.

This device is very common on BMCs to route a different channel to each
PCIe i2c bus downstream from the BMC.

Signed-off-by: Patrick Venture <venture@google.com>
Reviewed-by: Hao Wu <wuhaotsh@google.com>
Reviewed-by: Havard Skinnemoen <hskinnemoen@google.com>
Message-Id: <20210412194522.664594-5-venture@google.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>