x86: Fix Xen boot on 8-node AMD machines

x86: Fix Xen boot on 8-node AMD machines

In xen/arch/x86/cpu/amd.c the function disable_c1_ramping iterates
over the northbridges using NR_CPUS as an upper bound for the number of
nodes. If there are no more northbridges found, it stops iterating.
Sadly it just adds the northbridge number to 0x18 and uses this as a
PCI device number, so probing the 9th northbridge on an 8 node system will
be caught by the newly inserted assertion in pci_conf_read in current
unstable. We fix this by first querying the number of nodes from the
first northbridge to avoid the overflow.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Verified by Mark.Johnson@sun.com