arm: dts: Introduce k3-serdes.h from v6.6-rc1

Introduce the new serdes header from kernel v6.6-rc1

The DTS uses constants for SERDES MUX idle state values which were earlier
provided as bindings header. But they are unsuitable for bindings.
So move these constants in a header next to DTS.

Signed-off-by: Nishanth Menon <nm@ti.com>