genxml: Fix parsing of address fields in groups.

genxml: Fix parsing of address fields in groups.

For example,

    <group count="4" start="64" size="64">
      <field name="Pointer" start="5" end="63" type="address"/>
    </group>

used to generate:

   const uint64_t v2_address =
      __gen_combine_address(data, &dw[2], values->Pointer, 0);
   ...
   const uint64_t v4_address =
      __gen_combine_address(data, &dw[4], values->Pointer, 0);
   ...

but now generates code with proper subscripts:

   const uint64_t v2_address =
      __gen_combine_address(data, &dw[2], values->Pointer[0], 0);
   ...
   const uint64_t v4_address =
      __gen_combine_address(data, &dw[4], values->Pointer[1], 0);
   ...

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>