powerpc: converted embedded platforms to use new define_machine support

Removed platform_init usage on 83xx and 85xx and use define_machine and
probe().  For now we always return true in the problem since you can only
build for one specific board at a time.  This is an artificial constraint.
When we get ride of it we will need to update the Kconfig's for these
sub-arch's and make the board's probe() functions actually do something.

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