dt-bindings: PCI: Add bindings for brcmstb's PCIe device

The DT bindings description of the brcmstb PCIe device is described.
This node can only be used for now on the Raspberry Pi 4.

Signed-off-by: Jim Quinlan <james.quinlan@broadcom.com>
Co-developed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>