hwrng: omap - use devm_clk_get() instead of of_clk_get()

The omap-rng driver currently uses of_clk_get() to get a reference to
the clock, but never releases that reference. This commit fixes that by
using devm_clk_get() instead.

Fixes: 38321242 ("hwrng: omap - Add device variant for SafeXcel IP-76 found in Armada 8K")
Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>