ARM: dts: artpec: add pcie support

Add PCIe support to the ARTPEC-6 SoC. This uses the existing
pcie-artpec6 driver.
So, all that is needed is device tree entries in the DTS.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Jesper Nilsson <jespern@axis.com>