serial: 8250: 8250_omap: Support native RS485

serial: 8250: 8250_omap: Support native RS485

Recent TI Sitara SoCs such as AM64/AM65 have gained the ability to
automatically assert RTS when data is transmitted, obviating the need
to emulate this functionality in software.

The feature is controlled through new DIR_EN and DIR_POL bits in the
Mode Definition Register 3.  For details see page 8783 and 8890 of the
AM65 TRM:  https://www.ti.com/lit/ug/spruid7e/spruid7e.pdf

Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: Su Bao Cheng <baocheng.su@siemens.com>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Bin Liu <b-liu@ti.com>
Tested-by: Zeng Chao <chao.zeng@siemens.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Link: https://lore.kernel.org/r/e9f25f5c9200a35d3162973c2b45d6b892cc9bf2.1665906869.git.lukas@wunner.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>