powerpc/4xx: Create 4xx pseudo-platform in platforms/4xx

We have a lot of code in sysdev for supporting 4xx, ie. either 40x or
44x. Instead it would be cleaner if it was all in platforms/4xx.

This is slightly odd in that we don't actually define any machines in
the 4xx platform, as is usual for a platform directory. But still it
seems like a better result to have all this related code in a directory
by itself.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>