dt-binding: hwrng: add NPCM RNG documentation

Added device tree binding documentation for Nuvoton BMC
NPCM Random Number Generator (RNG).
Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/nuvoton,npcm-rng.txt

+NPCM SoC Random Number Generator
+
+Required properties:
+- compatible  : "nuvoton,npcm750-rng" for the NPCM7XX BMC.
+- reg         : Specifies physical base address and size of the registers.
+
+Example:
+
+rng: rng@f000b000 {
+	compatible = "nuvoton,npcm750-rng";
+	reg = <0xf000b000 0x8>;
+};