dt-bindings: PCI: meson: add DT bindings for Amlogic Meson PCIe controller

The Amlogic Meson PCIe host controller is based on the Synopsys DesignWare
PCI core. This patch adds documentation for the DT bindings in Meson PCIe
controller.

Signed-off-by: Yue Wang <yue.wang@amlogic.com>
Signed-off-by: Hanjie Lin <hanjie.lin@amlogic.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>