crypto: brcm - DT documentation for Broadcom SPU hardware

Device tree documentation for Broadcom Secure Processing Unit
(SPU) crypto hardware.

Signed-off-by: Steve Lin <[e-mail]>
Signed-off-by: Rob Rice <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

Documentation/devicetree/bindings/crypto/brcm,spu-crypto.txt

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+The Broadcom Secure Processing Unit (SPU) hardware supports symmetric
+cryptographic offload for Broadcom SoCs. A SoC may have multiple SPU hardware
+blocks.
+
+Required properties:
+- compatible: Should be one of the following:
+  brcm,spum-crypto - for devices with SPU-M hardware
+  brcm,spu2-crypto - for devices with SPU2 hardware
+  brcm,spu2-v2-crypto - for devices with enhanced SPU2 hardware features like SHA3
+  and Rabin Fingerprint support
+  brcm,spum-nsp-crypto - for the Northstar Plus variant of the SPU-M hardware
+
+- reg: Should contain SPU registers location and length.
+- mboxes: The mailbox channel to be used to communicate with the SPU.
+  Mailbox channels correspond to DMA rings on the device.
+
+Example:
+	crypto@612d0000 {
+		compatible = "brcm,spum-crypto";
+		reg = <0 0x612d0000 0 0x900>;
+		mboxes = <&pdc0 0>;
+	};