dt-bindings: rng: Document Freescale i.MX21 and i.MX31 RNGA compatibles

Freescale i.MX21 and i.MX31 SoCs contain a Random Number Generator
Accelerator module (RNGA), which is replaced by RNGB and RNGC modules
on later i.MX SoC series, the change adds a new compatible property
to describe the controller.

Since all versions of Freescale RNG modules are legacy, apparently
the documentation file has no more potential for further extensions,
nevertheless generalize it by removing explicit RNGC specifics.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>