powerpc/boot: Remove core support for Marvell mv64x60 hostbridges

There are no longer any platforms that use Marvell's mv64x60
hostbridges so remove the supporting boot code.

Signed-off-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>