crypto: omap-sham - use runtime_pm autosuspend for clock handling

Calling runtime PM API for every block causes serious performance hit to
crypto operations that are done on a long buffer. As crypto is performed
on a page boundary, encrypting large buffers can cause a series of crypto
operations divided by page. The runtime PM API is also called those many
times.

Convert the driver to use runtime_pm autosuspend instead, with a default
timeout value of 1 second. This results in upto ~50% speedup.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>