serial: mps2-uart: support combined irq

It turns out that some designs went for implementing only combined
interrupt for rx, tx and overrun, which is currently not supported
by the driver.  Support of combined irq is built on top of existent
irq handlers and activated automatically if only single irq was
specified in device tree.

Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>