hwrng: timeriomem - update to support more than one device

timeriomem_rng only supports a single device instance.  This patch
enables multiple timeriomem_rng devices to coexist as well as adds
some additional error checking.

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>