powerpc: Replace cpu_up/down with add/remove_cpu

The core device API performs extra housekeeping bits that are missing
from directly calling cpu_up/down.

See commit a6717c01 ("powerpc/rtas: use device model APIs and
serialization during LPM") for an example description of what might go
wrong.

This also prepares to make cpu_up/down a private interface for anything
but the cpu subsystem.

Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
CC: Benjamin Herrenschmidt <benh@kernel.crashing.org>
CC: Paul Mackerras <paulus@samba.org>
CC: Michael Ellerman <mpe@ellerman.id.au>
CC: Enrico Weigelt <info@metux.net>
CC: Ram Pai <linuxram@us.ibm.com>
CC: Nicholas Piggin <npiggin@gmail.com>
CC: Thiago Jung Bauermann <bauerman@linux.ibm.com>
CC: Christophe Leroy <christophe.leroy@c-s.fr>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: linuxppc-dev@lists.ozlabs.org
CC: linux-kernel@vger.kernel.org