Uboot RNG Driver using Data Co-processor

This commit introduces Random number generator to uboot. It uses DCP
driver for number generation.
RNG driver can be invoked by using below command on uboot prompt:-
           rng <number of bytes>

Signed-off-by: Kshitiz Varshney <kshitiz.varshney@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>