tty: Introduce software RS485 direction control support

[Matwey V. Kornilov]

Changes from v3:
 - Completely redesigned.
Changes from v2:
 - Introduced SER_RS485_SOFTWARE to show that software implementation is being used
 - serial8250_rs485_config is located as required
 - Timers are used to implement delay_before and delay_after timeouts
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