powerpc: Add 82xx/83xx/86xx to 6xx Multiplatform

There isn't any reason at this point that we can't build 82xx, 83xx & 86xx
support in with the other 6xx based boards.  Twiddle the Kconfigs to allow
this.

This allows us to remove the machine type selection for related to 6xx.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>