powerpc/pseries: Fix plpks crash on non-pseries

As reported[1] by Nathan, the recently added plpks driver will crash if
it's built into the kernel and booted on a non-pseries machine, eg
powernv:

  kernel BUG at arch/powerpc/kernel/syscall.c:39!
  Oops: Exception in kernel mode, sig: 5 [#1]
  LE PAGE_SIZE=64K MMU=Radix SMP NR_CPUS=2048 NUMA PowerNV
  ...
  NIP system_call_exception+0x90/0x3d0
  LR  system_call_common+0xec/0x250
  Call Trace:
    0xc0000000035c3e10 (unreliable)
    system_call_common+0xec/0x250
  --- interrupt: c00 at plpar_hcall+0x38/0x60
  NIP:  c0000000000e4300 LR: c00000000202945c CTR: 0000000000000000
  REGS: c0000000035c3e80 TRAP: 0c00   Not tainted  (6.0.0-rc4)
  MSR:  9000000002009033 <SF,HV,VEC,EE,ME,IR,DR,RI,LE>  CR: 28000284  XER: 00000000
  ...
  NIP plpar_hcall+0x38/0x60
  LR  pseries_plpks_init+0x64/0x23c
  --- interrupt: c00

On powernv Linux is the hypervisor, so a hypercall just ends up going to
the syscall path, which BUGs if the syscall (hypercall) didn't come from
userspace.

The fix is simply to not probe the plpks driver on non-pseries machines.

[1] https://lore.kernel.org/linuxppc-dev/Yxe06fbq18Wv9y3W@dev-arch.thelio-3990X/

Fixes: 2454a7af

 ("powerpc/pseries: define driver for Platform KeyStore")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Tested-by: Dan Horák <dan@danny.cz>
Reviewed-by: Dan Horák <dan@danny.cz>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/20220907065038.1604504-1-mpe@ellerman.id.au