Add PCIe EP mode dt-bindings for TI's J721E SoC.

Link: https://lore.kernel.org/r/20200722110317.4744-13-kishon@ti.comSigned-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>