hwrng: atmel - use clk_prepapre_enable/_disable_unprepare

Use clk_prepare_enable/_disable_unprepare instead of clk_enable/disable
to work properly with the CCF.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>