kernel: Move to Linux 4.15.6-openpower1

This moves us to the 4.15 kernel series. The defconfig has been
refreshed, with the major difference being NVMe multipath support is now
enabled in the kernel.

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 6b416f0..897d708 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 1/4] xhci: Reset controller on xhci shutdown
+Subject: [PATCH 1/3] 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.
@@ -14,10 +14,10 @@
  1 file changed, 1 insertion(+)
 
 diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
-index e5677700dea4..bde3e3e30d3b 100644
+index 5c1326154e66..894f1879df53 100644
 --- a/drivers/usb/host/xhci.c
 +++ b/drivers/usb/host/xhci.c
-@@ -729,6 +729,7 @@ static void xhci_shutdown(struct usb_hcd *hcd)
+@@ -722,6 +722,7 @@ static void xhci_shutdown(struct usb_hcd *hcd)
  	/* Yet another workaround for spurious wakeups at shutdown with HSW */
  	if (xhci->quirks & XHCI_SPURIOUS_WAKEUP)
  		pci_set_power_state(to_pci_dev(hcd->self.sysdev), PCI_D3hot);