dt-bindings: display: xlnx: Add maximum FRL rate property

commit  80e9bd5ea96b8affb8511c7708e2d48927fbf07c from
https://github.com/Xilinx/linux-xlnx.git

 xlnx_rebase_v5.10

Max FRL rate property is used in FRL mode operation of HDMI 2.1 Tx
driver.

Signed-off-by: Venkateshwar Rao Gannavarapu <venkateshwar.rao.gannavarapu@xilinx.com>
State: pending
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>