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>
Acked-by: Peter Rosin <peda@axentia.se>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210310112745.3445-4-kishon@ti.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>