hwrng: amd - remove reference to removed PPC_MAPLE config

Commit 62f8f307 ("powerpc/64: Remove maple platform") removes the
PPC_MAPLE config as a consequence of the platform’s removal.

The config definition of HW_RANDOM_AMD refers to this removed config option
in its dependencies.

Remove the reference to the removed config option.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>