hwrng: mxc-rnga - Use devm_clk_get_enabled() helpers

hwrng: mxc-rnga - Use devm_clk_get_enabled() helpers

The devm_clk_get_enabled() helpers:
    - call devm_clk_get()
    - call clk_prepare_enable() and register what is needed in order to
     call clk_disable_unprepare() when needed, as a managed resource.

This simplifies the code and avoids the calls to clk_disable_unprepare().

Signed-off-by: Huan Yang <link@vivo.com>
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>