Added additional pins for multi extruder setup

Also fixed an odd and possible erroneous  "PIN_EMAC_EREFCK_GPIO" pin
definition. Not sure if this actually caused any bugs though
Allow the operation of multiple MCP4461s at once

Changes required to allow for the two Digipot chips on the same I2C bus.
Merge pull request #1 from T3P3/Non_Due_Pin_Update

Added additional pins for multi extruder setup
Merge pull request #2 from T3P3/MCP4461_update

Allow the operation of multiple MCP4461s at once