dt-bindings: PCI: ti,j721e: Add binding to represent refclk to the connector

Add binding to represent refclk to the PCIe connector.

Link: https://lore.kernel.org/r/20210308063550.6227-2-kishon@ti.com


Signed-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>