powerpc: Use CONFIG_ARCH_HAS_FAST_MULTIPLIER

I ran some tests to compare hash_64 using shifts and multiplies.
The results:

POWER6:	~2x slower
POWER7: ~2x faster
POWER8: ~2x faster

Now we have a proper config option, select
CONFIG_ARCH_HAS_FAST_MULTIPLIER on POWER7 and POWER8.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>