x86: UV BAU and nodes with no memory

This patch fixes BAU initialization for systems containing
nodes with no memory and for systems with non-consecutive
node numbers.

Fixes and clarifies situations where pnode should be used instead
of node id.

Tested on the UV hardware simulator.

Signed-off-by: Cliff Wickman <cpw@sgi.com>
LKML-Reference: <E1LpjX3-00007N-12@eag09.americas.sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>