[i2c_mux] change i2c mux to power 1.8V and accept 3.3V
use low-voltage I2C mux powered from 1.8V that accepts pull-ups to 3.3V on secondary side or use PCA9306DCUR like in the original PXIe design to translave 1.8V to 3.3V I2C (see corresponding note for fpga-bank-87-88.SchDoc).