ARM: dts: artpec: add disabled node for PCIe endpoint mode

The PCIe controller in the artpec6 SoC supports both root complex and
endpoint mode, however, the controller can only be used in one of the
modes.

Both pci nodes are disabled by default. A DTS file can enable one of
them, depending on what mode it wants to run.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>