serdev: add helpers for cts and rts handling

Add serdev helper functions for handling of cts and rts
lines using the serdev's tiocm functions.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>