bindings: pcie: add pci-host-ecam-generic bindings

Add bindings for a generic PCIe Controller in ECAM mode.

ECAM stands for PCI Express Enhanced Configuration Access Mechanism
where the PCIe config space is flat memory-mapped.

An optional msi-parent phandle is added to link to an optional
MSI/MSI-X Interrupt Message Translation HW component.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>