target-ppc: disable unsupported modes for SPR_CTRL/SPR_UCTRL

The Figure 17 "SPR encodings" of the PowerISA 2.07 describes CTRL SPR as:

                       priviledged
#   spr5-9 spr0-4 name mtspr mfspr len cat
136 00100  01000  CTRL   -    no    32  S
152 00100  11000  CTRL  yes    -    32  S

According to this chart, the hypervisor's CTRL (#152) does not support
reading, the user-space's CTRL (UCTRL, #136) does not support writing.

This replaces unsupported operations with the default SPR_NOACCESS hook.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alexander Graf <agraf@suse.de>
prep: Drop from ppcemb-softmmu

ppcemb covers only embedded processors, which does not include PReP.

Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Signed-off-by: Alexander Graf <agraf@suse.de>