ARM: 6092/1: atmel_serial: support for RS485 communications

Final version of the patch that adds support for RS485 communications to the atmel_serial driver.

The patch has been already sent and discussed on both linux-kernel and linux-arm-kernel mailing lists several times.

Many people collaborated to improve and test the code:

Tested-by: Sebastian Heutling <Sebastian.Heutling@who-ing.de>
Tested-by: Bernhard Roth <br@pwrnet.de>
Reviewed-by: Ryan Mallon <ryan@bluewatersys.com>
Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
Signed-off-by: Michael Trimarchi <michael@evidence.eu.com>
Signed-off-by: Rick Bronson <rick@efn.org>
Signed-off-by: Sebastian Heutling <Sebastian.Heutling@who-ing.de>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>