arm: dts: k3*: Use local header for SERDES MUX idle-state values

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.

NOTE: sync with v6.6-rc1 will bring in this change naturally.

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