crypto: atmel - use devm_clk_get_prepared() helpers

Simplify the code by replacing devm_clk_get() and clk_prepare() with
devm_clk_get_prepared(), which also avoids the call to clk_unprepare().

Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>