witherspoon: Squash spurious I2C errors

On witherspoon there's an I2C bus connecting the each chip to the GPUs
connected to that chip via NVLink. That bus has a 750kOhm pullup on the
system planar with prevents the bus from operating correctly.

Each GPU has a smaller pullup which makes the bus usable when a GPU is
plugged in, but on systems without GPUs we get a lot of spurious I2C
master errors. Specificly, because of the oversized pullup the SDA and
SCL for that bus cannot return to '1' fast enough, so the master assumes
that another master is driving the bus and that it should stop.

Work around this by disabling the affected port when there's no GPUs
detected in the system.

Reviewed-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Oliver O'Halloran <oohall@gmail.com>

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