crypto: brcm - DT documentation for Broadcom SPU hardware

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

Signed-off-by: Steve Lin <steven.lin1@broadcom.com>
Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>