dt-bindings: PCI: keystone: Add "reg-names" binding information

Add "reg-names" binding information in order for device tree node
to be populated with the correct register strings.

This will break old DT compatibility. However Keystone PCI has never
worked in upstream kernel due to lack of SERDES support, so, before
SERDES support is added, cleanup the Keystone PCI dt-bindings.

This new binding will also be used by PCI in AM654 platform.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>