hwrng: imx-rngc - enable driver for i.MX6

i.MX6 SL, SLL, ULL, ULZ SoCs have an RNGB block.

Since imx-rngc driver supports also rngb,
let's enable it for these SoCs too.

Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Reviewed-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>