bindings: PCI: artpec: Add support for the ARTPEC-7 SoC

Add support for the ARTPEC-7 SoC in the artpec6 driver.
The ARTPEC-6 SoC and the ARTPEC-7 SoC are very similar.
Unfortunately, some fields in the PCIECFG and PCIESTAT
register have changed.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Rob Herring <robh@kernel.org>