Move RS485 implementation from drivers to serial core (v3)

[Lino Sanfilippo]

This patch series is an attempt to simplify rs485 implementation in drivers
by moving the following tasks out of the drivers into the serial core:

- ensure sane RTS settings: in case of an invalid configuration (both RTS
  after send and RTS on send set or both unset) enable RTS on send and
  disable RTS after send

- nullify the padding field of the serial_rs485 struct before it is
  returned to userspace

- copy the configuration stored in the serial_rs485 struct to the port
  configuration if setting the configuration in the driver was successfull

- limit the RTS delay to 100ms


Redundant code has been removed from the following drivers for now:

- atmel
- fsl_lpuart
- amba
- imx
- max310x
- omap-serial
- sc16is7xx
- stm32-usart

The code has been tested with the amba pl011 driver. This series applies
against Gregs tty-testing branch.

Changes in v2:
- use a makro for max RTS delays and comment it (as requested by Jiri)
- add a comment concerning the memset of a structures padding field
- correct typos in the commit message (found by Uwe) 
- rephrase all commit messages to make more clear that function 
  uart_set_rs485_config() has been extended by checks and other
  functionalities (as requested by Uwe)

Changes in v3:
- add warning messages if the serial core corrects RS485 values (as requested by Lukas Wunner)
- dont expose the macro for max RTS delays to userspace (as requested by Greg)