[MPCore Watchdog]: Convert from misc_dev to dynamic device node.

[Peter Fordham]

The framework looks great. I hope it gets in soon.

> If you use a bus then the device creation is slightly more involved than
> with a class - create the bus, allocate the device and assign the
> bus_type then set the name with dev_set_name().  That should be enough
> for udev to work out the name of the device.

It's a platform device so it's already on the platform bus under:

/sys/devices/platform/mpcore_wdt

Do I need to create another bus? The platform driver name is used in driver
matching so it doesn't seem like the right thing to do to change it to watchdog.

-Pete