serial: sa1100: add note about modem control signals

As suggested by Uwe, add a note indicating that the modem control
signals do not support interrupts, which precludes the driver from
using mctrl_gpio_init().

Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>