serial: mxs-auart: Remove serial_mxs_probe_dt()

The mxs platform is devicetree-only, so there is no need to check
whether it was instantiated via devicetree.

Simplify the code my removing serial_mxs_probe_dt() and add its
content into the main probe function.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20210118152154.1644569-1-festevam@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>