Convert the OMAP onboard hardware RNG driver to use runtime PM.

This allows us to remove some OMAP-specific cpu_is_omap*() calls from
the RNG driver.
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>