pldmbios: Adding a BIOS attribute that can limit the number of cores activated

Adding attribute "hb_field_core_override" that can be used to limit
the number of cores HB activates. The default is 0 which is interpreted
as to activate all cores.  If given value N which is not 0, then
for a fused core system 2N cores will be activated.  For a non-fused
core system then N cores will be activated.

This is useful if a customer only wishes to activate N cores and not
activate all cores available.

Signed-off-by: Roland Veloz <rveloz@us.ibm.com>
Change-Id: Icca120b445810e1b7b8d41f0699ef87a4f0b6037