Enable the second and the third I2C controllers on Beaglebone-black.

The first I2C controller is only used to manage the on-board devices (PMIC
and HDMI framer) and its bus is not exposed on the expasion headers.

With this change the following pins on the P9 expansion headers are now
reserved as I2C pins:

 Pin 17 - I2C1 SCL
 Pin 18 - I2C1 SDA
 Pin 19 - I2C2 SCL
 Pin 20 - I2C2 SDA

The I2C2 is the bus that should be used to read the contents of cape
eeproms.

Approved by:	adrian (mentor, implicit)