Implement the previously empty mpfs_ddr_rand with adapted "seiran128" code from https://github.com/andanteyk/prng-seiran This implements a non-secure prng, which is minimal in size. The DDR training doesn't need cryptographically secure prng, and linking in the NuttX crypto would increase the code size significantly for bootloaders. Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>