swift: Add si7021a20 ambient sensor

Swift has 3 ambient temperature sensors, the si7021a20 will be labeled
as `ambient2` and initially use the same warn/crit temperature
configurations as Witherspoon.

Tested:
    Built image that contained the config file

Change-Id: I6c5e9924c2204113e1caa6109b48142321f449c7
Signed-off-by: Matthew Barth <msbarth@us.ibm.com>
2 files changed