tty: serial: allow ports to override the irq handler

Some serial ports may have unusal requirements for interrupt handling
(e.g. the Synopsys DesignWare 8250-alike port and it's busy detect
interrupt).  Add a .handle_irq callback that can be used for platforms
to override the interrupt behaviour in a similar fashion to the
.serial_out and .serial_in callbacks.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>