dt-bindings: ti-serdes: Deprecate header with constants with v6.6-rc1

The constants to define the idle state of SERDES MUX were defined in
bindings header. They are used only in DTS and driver uses the dt property
to set the idle state making it unsuitable for bindings.
The constants are moved to header next to DTS ("arch/arm/boot/dts/")
and all the references to bindings header are removed.
So add a warning to mark this bindings header as deprecated.

We could probably drop this header, but let us wait for kernel to
cleanup.

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