[POWERPC] Add starting of secondary 86xx CPUs.

Clear the high BATS during load_up_mmu if FTR_HAS_HIGH_BATS.
Allow just a bit more time for secondary CPUs to phone home.

Signed-off-by: Wei Zhang <Wei.Zhang@freescale.com>
Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>