serial: sa1100: add support for mctrl gpios

Add support for the generic mctrl gpio helper.  This will allow us to
convert several board files to use the gpiod tables to assign GPIOs to
serial ports, rather than needing to have private function callbacks.

If the generic mctrl gpio helper fails, ignore the mctrl gpios rather
than preventing the (possibly console) serial port from being created.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>