Add Reliable Asynchronous Transfer Protocol ...

This patch adds support for Reliable Asynchronous Transfer Protocol (RATP)
as described in RFC916.

Communication over RS232 is often unreliable as characters are lost or
misinterpreted. This protocol allows for a reliable packet based communication
over serial lines.

The implementation simply follows the state machine described in the RFC
text with one exception. RFC916 uses a plain checksum for the
transferred data. We decided to use CRC16 for greater robustness. Since
this is the only RFC916 implementation we currently know of interoperability
with other implementations should not matter.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Andrey Smirnov <andrew.smirnov@gmail.com>