arm64: dts: ti: k3-j721e-common-proc-board: Use external clock for SERDES

Use external clock for all the SERDES used by PCIe controller. This will
make the same clock used by the local SERDES as well as the clock
provided to the PCIe connector.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210603143427.28735-4-kishon@ti.com