powerpc/85xx: Fix oops when CONFIG_FSL_PMC=n

powerpc/85xx: Fix oops when CONFIG_FSL_PMC=n

When CONFIG_FSL_PMC is set to n, no value is assigned to cpu_up_prepare
in the mpc85xx_pm_ops structure. As a result, oops is triggered in
smp_85xx_start_cpu().

  smp: Bringing up secondary CPUs ...
  kernel tried to execute user page (0) - exploit attempt? (uid: 0)
  BUG: Unable to handle kernel instruction fetch (NULL pointer?)
  Faulting instruction address: 0x00000000
  Oops: Kernel access of bad area, sig: 11 [#1]
  ...
  NIP [00000000] 0x0
  LR [c0021d2c] smp_85xx_kick_cpu+0xe8/0x568
  Call Trace:
  [c1051da8] [c0021cb8] smp_85xx_kick_cpu+0x74/0x568 (unreliable)
  [c1051de8] [c0011460] __cpu_up+0xc0/0x228
  [c1051e18] [c0031bbc] bringup_cpu+0x30/0x224
  [c1051e48] [c0031f3c] cpu_up.constprop.0+0x180/0x33c
  [c1051e88] [c00322e8] bringup_nonboot_cpus+0x88/0xc8
  [c1051eb8] [c07e67bc] smp_init+0x30/0x78
  [c1051ed8] [c07d9e28] kernel_init_freeable+0x118/0x2a8
  [c1051f18] [c00032d8] kernel_init+0x14/0x124
  [c1051f38] [c0010278] ret_from_kernel_thread+0x14/0x1c

Fixes: c45361abb918 ("powerpc/85xx: fix timebase sync issue when CONFIG_HOTPLUG_CPU=n")
Reported-by: Martin Kennedy <hurricos@gmail.com>
Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Tested-by: Martin Kennedy <hurricos@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211126041153.16926-1-nixiaoming@huawei.com