target-ppc: Enable frsqrtes on Power7 and Power8

The frsqrtes instruction was introduced prior to ISA 2.06 and is
support on both the Power7 and Power8 processors.  However, this
instruction is handled as illegal in the current QEMU emulation
machines.  This patch enables the existing implemention of frsqrtes
in the P7 and P8 machines.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Alexander Graf <agraf@suse.de>
