kernel: Update to Linux 4.14.20-openpower1

This adds an upstream backport of a fix for the Broadcom network device:

    tg3: APE heartbeat changes

Signed-off-by: Joel Stanley <joel@jms.id.au>
diff --git a/openpower/linux/0001-xhci-Reset-controller-on-xhci-shutdown.patch b/openpower/linux/0001-xhci-Reset-controller-on-xhci-shutdown.patch
index a0724db..6b416f0 100644
--- a/openpower/linux/0001-xhci-Reset-controller-on-xhci-shutdown.patch
+++ b/openpower/linux/0001-xhci-Reset-controller-on-xhci-shutdown.patch
@@ -1,7 +1,7 @@
 From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
 From: Brian King <brking@linux.vnet.ibm.com>
 Date: Wed, 25 Oct 2017 10:42:59 +1100
-Subject: [PATCH 1/3] xhci: Reset controller on xhci shutdown
+Subject: [PATCH 1/4] xhci: Reset controller on xhci shutdown
 
 Fixes kexec boot. Without a hard reset, some USB chips will fail to
 initialize in a kexec booted kernel.