fwohci: report whether PhysicalUpperBound register is implemented

Please see section 5.15 of 1394 OHCI Specification.
If the register is not implemented, then the physical response unit is
limited to the first 4GB of the physical memory.
In that case the non-cooperative debugging over firewire (using /dev/fwmem)
can not be expected to work if a target has more RAM than that.
The method is described in gdb.4 and the Developer's Handbook.

It seems that most of the consumer hardware does not implement
PhysicalUpperBound register.

MFC after: 1 week