powerpc/pseries: Cleanup best_energy_hcall detection

Currently we search for the best_energy hcall using a custom function.  Move
this to using the firmware_feature_table.

Signed-off-by: Michael Neuling <mikey@neuling.org>
cc: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
cc: Linux PPC dev <linuxppc-dev@ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>