serial: 8250_bcm2835aux: Add support for RTS/CTS auto flow control

The bcm2835aux mini UART can automatically de-assert RTS based on the RX
FIFO fill level and gate the transmitter on CTS, but the driver never
made use of it. Wire it up:

- Enable AUTORTS/AUTOCTS (with the RTS4 threshold and inverted, i.e.
  conventional active-low, polarity) whenever CRTSCTS is set on ports
  declaring the "uart-has-rtscts" property, advertised via
  UPF_HARD_FLOW.

- Gate the AUTORTS function on TIOCM_RTS in set_mctrl() so that clearing
  RTS (port close, TIOCMBIC, B0) actually de-asserts the pin while auto
  flow control is active.

- Implement throttle()/unthrottle() by disabling the RX interrupts and
  letting the FIFO fill up, making the hardware de-assert RTS - the same
  approach as in 8250_omap. Re-sync this state on termios and rs485
  changes, as those paths may re-enable the RX interrupts (or leave them
  disabled) behind the throttle's back.

- Never enable auto flow control together with rs485, which uses RTS as
  the transceiver direction control. As the software CTS fallback of the
  serial core cannot work without a modem status interrupt, strip
  CRTSCTS whenever it cannot be handled in hardware. Keep it when a CTS
  GPIO is present, as the serial core's mctrl_gpio handling then
  delivers the interrupt and the software fallback keeps working.

- Restore RXEN when rs485 is disabled, as the emulation may be torn down
  without the rs485_stop_tx() callback ever being called, which would
  leave the receiver disabled.

Assisted-by: Claude:claude-fable-5
Signed-off-by: Jan Čermák <sairon@sairon.cz>
Link: https://patch.msgid.link/20261001090432.1045270-1-sairon@sairon.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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