clearfog: Enable kernel's PMBus

...because these will be needed by the PSU monitoring if everything goes
according to my expectations.

Change-Id: I3fa12dd101e0959d10cf1559b3e987c26734d1f0
diff --git a/board/czechlight/clearfog/linux.fragment b/board/czechlight/clearfog/linux.fragment
index b7ed5b0..1f60709 100644
--- a/board/czechlight/clearfog/linux.fragment
+++ b/board/czechlight/clearfog/linux.fragment
@@ -1 +1,6 @@
+# this is a part of the Clearfog Base carrier
 CONFIG_SENSORS_MCP3021=y
+
+# PMBus for the PSUs
+CONFIG_PMBUS=y
+CONFIG_SENSORS_PMBUS=y