iwlwifi: pcie: retrieve and parse ACPI power limitations

Some platforms may have power limitations on PCIe cards connected to
specific root ports.

This information is encoded as part of the ACPI tables, for instance:
<snip>
           Name (SPLX, Package (0x02)
           {
               Zero,
               Package (0x03)
               {
                   0x07,
                   0x00000500,
                   0x80000000
               }
           })

           Method (SPLC, 0, Serialized)
           {
               Return (SPLX)
           }
</snip>

The structure returned contains the domain type, the default power
limitation and the default time window (reserved for future use).

Upon PCI probing, call the relevant ACPI method, parse the returned
structure, and save the power limitation.

Signed-off-by: Ido Yariv <idox.yariv@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>