serial: mxs-auart: add interrupts for modem control lines

Handle CTS/DSR/RI/DCD GPIO interrupts in mxs-auart.

Signed-off-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>