dt-bindings: PCI: designware: Add support for EP in DesignWare driver

Add device tree binding documentation for the EP in PCIe DesignWare driver.

Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
[lorenzo.pieralisi@arm.com: updated commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>