Add a serdev controller driver for tty ports.

The controller is registered with serdev when tty ports are registered
with the TTY core. As the TTY core is built-in only, this has the side
effect of making serdev built-in as well.
Signed-off-by: NRob Herring <robh@kernel.org>
Reviewed-By: NSebastian Reichel <sre@kernel.org>
Tested-By: NSebastian Reichel <sre@kernel.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>