target-ppc: fix Authority Mask Register init value

The existing default value (-1) of the AMR register forbids data access
to all 32 classes. Since the guest linux does not change this register,
we end up with the guest hanging right after switching from the real to
protected mode.

This sets the default AMR value to zero what enables data access for all
classes.

The only reason for not hitting this bug before is that
kvm_arch_put_registers() did not put any SPR to KVM due to missing
assignment of @one_reg_id in _spr_register() (which is going to be fixed
by a separate patch).

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Greg Kurz <gkurz@linux.vnet.ibm.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
