Header I2C bus should have pull-up resistors

[nkrackow]

The I2C bus going to the CPU GPIO header currently doesn't have pull-up resistors. If the CPU internal pull-ups are not enabled this leads to a bus error if no mezzanine with a pull-up on the I2C lines is plugged in. But the CPU internal pull-ups are also too large for nominal I2C spec.

[gkasprow]

This was done on purpose because these pins are also GPIOs. I added pull-ups on the mezzanines already.

[jordens]

The use case is without a mezzanine, where we'd want to probe for mezzanine presence. The CPU GPIO pullups are so weak that they may cause spurious ACK.