hwrng: meson - use devm_clk_get_optional_enabled

Use devm_clk_get_optional_enabled() to simplify the code.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>