target-ppc: Remove check_pow_970FX

After merging 970s into one class, check_pow_970() is used for all of them.
Since POWER5+ is no different in the matter of supported power modes,
let's use the same check_pow() callback for POWER5+ too,

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
