hwrng: omap - Remove global definition of hwrng

The omap-rng driver currently assumes that there will only ever be a
single instance of an RNG device. For this reason, there is a statically
allocated struct hwrng, with a fixed name. However, registering two
struct hwrng with the same isn't accepted by the RNG framework, so we
need to switch to a dynamically allocated struct hwrng, each using a
different name. Then, we define the name of this hwrng to "dev_name(dev)",
so the name of the data structure is unique per device.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>