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: NVladimir Zapolskiy <vz@mleia.com>
Reviewed-by: NRob Herring <robh@kernel.org>
Reviewed-by: NFabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/imx-rngc.txt → Documentation/devicetree/bindings/rng/imx-rng.txt

-Freescale RNGC (Random Number Generator Version C)
-
-The driver also supports version B, which is mostly compatible
-to version C.
+Freescale RNGA/RNGB/RNGC (Random Number Generator Versions A, B and C)
 
 Required properties:
 - compatible : should be one of
+               "fsl,imx21-rnga"
+               "fsl,imx31-rnga" (backward compatible with "fsl,imx21-rnga")
                "fsl,imx25-rngb"
                "fsl,imx35-rngc"
 - reg : offset and length of the register set of this block
-- interrupts : the interrupt number for the RNGC block
-- clocks : the RNGC clk source
+- interrupts : the interrupt number for the RNG block
+- clocks : the RNG clk source
 
 Example: