serial: 8250_dwlib: RS485 HW half & full duplex support

serial: 8250_dwlib: RS485 HW half & full duplex support

The Synopsys DesignWare UART can be configured to have HW support for
the RS485 protocol from IP version 4.0 onward. Add support for
hardware-controlled half duplex and full duplex modes.

HW will take care of managing DE and RE, the driver just gives it
permission to use either by setting both to 1.

To ask for full duplex mode, userspace sets SER_RS485_RX_DURING_TX flag
and HW will take care of the rest.

Set delay_rts_before_send and delay_rts_after_send to zero for now. The
granularity of that ABI is too coarse to be useful.

Co-developed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Co-developed-by: Raymond Tan <raymond.tan@intel.com>
Signed-off-by: Raymond Tan <raymond.tan@intel.com>
Co-developed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220426122448.38997-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>