hw/ppc: Fix iothread locking in the 405 code

When using u-boot as firmware with the taihu board, QEMU aborts with
this assertion:

 ERROR:../accel/tcg/tcg-accel-ops.c:79:tcg_handle_interrupt: assertion failed:
  (qemu_mutex_iothread_locked())

Running QEMU with "-d in_asm" shows that the crash happens when writing
to SPR 0x3f2, so we are missing to lock the iothread in the code path
here.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20211006071140.565952-1-thuth@redhat.com>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>