target-ppc: MSR_POW not supported on POWER7/7+/8

Remove MSR_POW from the msr_mask for POWER7/7P/8.

Signed-off-by: Anton Blanchard <anton@samba.org>
Reviewed-by: Cédric Le Goater <clg@fr.ibm.com>
Tested-by: Cédric Le Goater <clg@fr.ibm.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>