Add support for 20MHz clock source

[bsundahl1]

A device that I am working with (Digilent 410-353 PMOD CAN CONTROLLER) uses a 20 MHz oscillator. The Arduino-CAN driver is working fine on this device (which uses a register compatible MCP25625 controller chip) except that all the baud rates are 25% high. This change adds another section to the config table to support the 20MHz oscillator rate.