x86/PCI: Correct SPDX comment style

Change:

  drivers/pci/controller/pcie-cadence.h
  drivers/pci/controller/pcie-rockchip.h

to use the correct SPDX comment style per section 2 of
Documentation/process/license-rules.rst.

These resolve the following checkpatch.pl warning:

  WARNING: Missing or malformed SPDX-License-Identifier tag in line 1

[bhelgaas: split to separate patch]
Link: https://lore.kernel.org/r/20190828135322.10370-1-kw@linux.com
Signed-off-by: Krzysztof Wilczynski <kw@linux.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/controller/pcie-cadence.h

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0
+/* SPDX-License-Identifier: GPL-2.0 */
 // Copyright (c) 2017 Cadence
 // Cadence PCIe controller driver.
 // Author: Cyrille Pitchen <cyrille.pitchen@free-electrons.com>

drivers/pci/controller/pcie-rockchip.h

@@ -1,4 +1,4 @@
-// SPDX-License-Identifier: GPL-2.0+
+/* SPDX-License-Identifier: GPL-2.0+ */
 /*
  * Rockchip AXI PCIe controller driver
  *