meta-obmc-machines: Add MAX31785 to Firestone and Witherspoon

These two machines contain the MAX31785 fan controller. We have an
interim driver for these in the dev-4.7 Aspeed kernel tree, so add the
driver to the configs for these machines.

Change-Id: Id619e8b6e3338c880aabb8780db468cc5fff6a6f
Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/meta-witherspoon/recipes-kernel/linux/linux-obmc/witherspoon.cfg b/meta-witherspoon/recipes-kernel/linux/linux-obmc/witherspoon.cfg
index 99efe89..01fb1c1 100644
--- a/meta-witherspoon/recipes-kernel/linux/linux-obmc/witherspoon.cfg
+++ b/meta-witherspoon/recipes-kernel/linux/linux-obmc/witherspoon.cfg
@@ -1,2 +1,3 @@
 CONFIG_HWMON=y
 CONFIG_SENSORS_TMP421=y
+CONFIG_SENSORS_MAX31785=y