hwrng: nomadik - use clk_prepare_enable()

The Nomadik HW RNG driver has seen some rust and is not preparing
the clock before use. Fix this up so we get rid of runtime
complaints from the clock subsystem.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>