dt-bindings: nvmem: Add bindings for ZynqMP nvmem driver

Add documentation to describe Xilinx ZynqMP nvmem driver
bindings.

Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>