crypto: picoxcell - Update to the current clk API

The picoXcell hardware crypto accelerator driver was using an
older version of the clk framework, and not (un)preparing the
clock before enabling/disabling it.  This change uses the handy
clk_prepare_enable function to interact with the current clk
framework correctly.

Signed-off-by: Michael van der Westhuizen <michael@smart-africa.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>