Automatically probe the board irq when no irq parameter is provided, to
simulate PnP. The old default behaviour was to disable the irq.

Update driver documentation accordingly and add some printk messages to
make this behaviour visible.
Signed-off-by: NFinn Thain <fthain@telegraphics.com.au>
Tested-by: NOndrej Zary <linux@rainbow-software.org>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>