PCI: j721e: Add support to provide refclk to PCIe connector

Add support to provide refclk to PCIe connector.

Link: https://lore.kernel.org/r/20210308063550.6227-5-kishon@ti.com


Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>