powerpc/mm/64s: Only use slbfee on CPUs that support it

The slbfee instruction was only added in ISA 2.05 (Power6), it's not
supported on older CPUs. We don't have a CPU feature for that ISA
version though, so just use the ISA 2.06 feature flag.

Fixes: e15a4fea ("powerpc/64s/hash: Add some SLB debugging tests")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>