powerpc: Move default hash table size calculation to hash_utils_64.c

We weren't computing the size of the hash table correctly on iSeries
because the relevant code in prom.c was #ifdef CONFIG_PPC_PSERIES.
This moves the code to hash_utils_64.c, makes it unconditional, and
cleans it up a bit.

Signed-off-by: Paul Mackerras <paulus@samba.org>