kernel: Move to Linux v4.19.0-openpower1

Move to a new upstream kernel release. This includes a buildroot bump to
support 4.19 kernels.

The defconfig has been regenerated against 4.19 and the non-default IO
schedulers disabled. No functional changes should result.

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 b577e6a..1b4f4bf 100644
--- a/openpower/linux/0001-xhci-Reset-controller-on-xhci-shutdown.patch
+++ b/openpower/linux/0001-xhci-Reset-controller-on-xhci-shutdown.patch
@@ -14,7 +14,7 @@
  1 file changed, 1 insertion(+)
 
 diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
-index c2220a7fc758..e0c42a75ceb8 100644
+index 0420eefa647a..618fabbdb4cc 100644
 --- a/drivers/usb/host/xhci.c
 +++ b/drivers/usb/host/xhci.c
 @@ -796,6 +796,7 @@ static void xhci_shutdown(struct usb_hcd *hcd)