hwrng: amd - enable AMD hw rnd driver for Maple PPC boards

PPC 970FX Evaluation kit (Maple) boards bear AMD8111 southbridge.
Allow this driver to be compiled in if PPC_MAPLE is selected.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>