hwrng: ba431 - HW_RANDOM_BA431 should not default to y

As HW_RANDOM_BA431 does not have any platform dependency, it should not
default to enabled.

Fixes: 0289e9be ("hwrng: ba431 - add support for BA431 hwrng")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>