dt-bindings: imx-rng: Allow passing only "fsl,imx31-rnga"

On imx31.dtsi the rng compatible string contains "fsl,imx31-rnga" only.

Adjust the binding to accept passing "fsl,imx31-rnga" only.

This fixes the following dt-schema warning:

imx31-lite.dtb: rng@53fb0000: compatible: 'oneOf' conditional failed, one must be fixed:
['fsl,imx31-rnga'] is too short
'fsl,imx21-rnga' was expected
'fsl,imx25-rngb' was expected
'fsl,imx31-rnga' is not one of ['fsl,imx6sl-rngb', 'fsl,imx6sll-rngb', 'fsl,imx6ull-rngb']
'fsl,imx35-rngc' was expected

Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>