Multidrop support

[Rodolfo Giometti <giometti@xxxxxxxxxxxx>]

Here are two patches to introduce multidrop support.

First one defines new termios bits useful to enable multidrop support,
while the second one effectively implements multidrop for the
Atmel/Microchip serial controllers.

Of course other serial drivers can do similar steps if the hardware
they control can do similar operations.

Rodolfo