hwrng: omap - Add device tree support

Add Device Tree suport to the omap-rng driver.
Currently, only support for OMAP2 and OMAP3 is
being added but support for OMAP4 and OMAP5 will
be added in a subsequent patch.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>