powerpc: Only save/restore SDR1 if in hypervisor mode

Currently, when not in hypervisor mode the kernel
Oopses during suspend or hibernation when accessing
the SDR1 register, because it is only available
in hypervisor mode.  Access to it needs to be
protected in BEGIN/END_FW_FTR_SECTION.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reported-by: Jimmy Pan <jipan@redhat.com>
Tested-by: Jimmy Pan <jipan@redhat.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>