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

Convert the driver to use autosuspend for runtime_pm. This boosts the
performance, and optimizes the power consumption for the driver. By
default, the timeout value for autosuspend is set to one second.

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