dt-bindings: rng: add RNGB compatibles for i.MX6 SoCs

RNGB block is found in some i.MX6 SoCs - 6SL, 6SLL, 6ULL, 6ULZ.
Add corresponding compatible strings.

Note:

Several NXP SoC from QorIQ family (P1010, P1023, P4080, P3041, P5020)
also have a RNGB, however it's part of the CAAM
(Cryptograhic Accelerator and Assurance Module) crypto accelerator.
In this case, RNGB is managed in the caam driver
(drivers/crypto/caam/), since it's tightly related to
the caam "job ring" interface, not to mention CAAM internally relying on
RNGB as source of randomness.

On the other hand, the i.MX6 SoCs with RNGB have a DCP
(Data Co-Processor) crypto accelerator and this block and RNGB
are independent.
Signed-off-by: Horia Geantă <horia.geanta@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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

@@ -5,6 +5,9 @@ Required properties:
                "fsl,imx21-rnga"
                "fsl,imx31-rnga" (backward compatible with "fsl,imx21-rnga")
                "fsl,imx25-rngb"
+               "fsl,imx6sl-rngb" (backward compatible with "fsl,imx25-rngb")
+               "fsl,imx6sll-rngb" (backward compatible with "fsl,imx25-rngb")
+               "fsl,imx6ull-rngb" (backward compatible with "fsl,imx25-rngb")
                "fsl,imx35-rngc"
 - reg : offset and length of the register set of this block
 - interrupts : the interrupt number for the RNG block