powerpc/85xx: Fix oops when mpc85xx_smp_guts_ids node cannot be found

When the field described in mpc85xx_smp_guts_ids[] is not configured in
dtb, the mpc85xx_setup_pmc() does not assign a value to the "guts"
variable. As a result, the oops is triggered when
mpc85xx_freeze_time_base() is executed.

Fixes: 56f1ba28 ("powerpc/mpc85xx: refactor the PM operations")
Cc: stable@vger.kernel.org # v4.6+
Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210929033646.39630-2-nixiaoming@huawei.com