hwrng: atmel - Add TRNG DT binding doc

Document DT bindings of Atmel's TRNG (True Random Number Generator) IP.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/hwrng/atmel-trng.txt

+Atmel TRNG (True Random Number Generator) block
+
+Required properties:
+- compatible : Should be "atmel,at91sam9g45-trng"
+- reg : Offset and length of the register set of this block
+- interrupts : the interrupt number for the TRNG block
+- clocks: should contain the TRNG clk source
+
+Example:
+
+trng@fffcc000 {
+	compatible = "atmel,at91sam9g45-trng";
+	reg = <0xfffcc000 0x4000>;
+	interrupts = <6 IRQ_TYPE_LEVEL_HIGH 0>;
+	clocks = <&trng_clk>;
+};