ppc4xx: Add option for PPC440SPe ports without old Rev. A support

The 440SPe Rev. A is quite old and newer 440SPe boards don't need support
for this CPU revision. Since removing support for this older version
simplifies the creation for newer U-Boot ports, this patch now enables
440SPe > Rev. A support by creating the CONFIG_440SPE_REVA define. By
defining this in the board config header, Rev. A will still be supported.
Otherwise (default for newer board ports), Rev. A will not be supported.

Signed-off-by: Stefan Roese <sr@denx.de>