Change wspoon GPU overtemp polling to 500ms

For the Witherspoon system, reduce the polling interval
for the GPU overtemp condition detected by the UCD90160
power sequencer from 1s to 500ms.

This is done to improve fault isolation, as well as HW safety,
as the the GPU will turn itself off after 1s of an overtemp
so the code may detect a PGOOD fault instead if it doesn't
catch it in time.

The GPU spec also says to shut off all power on an overtemp
as soon as possible to prevent HW damage caused by other
connected GPUs still trying to talk to the faulted one.
This change will shorten that time, as a hard power off is
already issued after an overtemp is detected.

Resolves openbmc/openbmc#2719

Change-Id: I1146abfd38e75ec9e9c9fd4f83fe3a9b68c0bd89
Signed-off-by: Matt Spinler <spinler@us.ibm.com>