add hardware handshake support to atmel_serial

Adds Hardware Handshake (aka RTS/CTS flow-control) support to
atmel_serial driver, as a termios flag.

For this to actually work, the platform code needs to configure the RTS
and CTS pins for use by the USART. This has been done for AVR32 as a
separate patch.

Signed-off-by: Peter Ma <pma@mediamatech.com>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
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