dt-bindings: ti-serdes-mux: Add defines for AM64 SoC

AM64 has a single lane SERDES which can be configured to be used
with either PCIe or USB. Define the possilbe values for the SERDES
function in AM64 SoC here.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210721155849.20994-7-kishon@ti.com