powerpc/trace: Restrict hash_fault trace event to HASH MMU

'perf list' on powerpc 8xx shows an event named "1:hash_fault".

This event is pointless because trace_hash_fault() is called only
from mm/book3s64/hash_utils.c

Only define it when CONFIG_PPC_64S_HASH_MMU is selected.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/85a86e51b4ab26ce4b592984cc0a0851a3cc9479.1708076780.git.christophe.leroy@csgroup.eu