subsys/random: Add Xoroshiro128+ PRNG

This adds an implementation of Xoroshiro128+ PRNG, based on the
original implementation at [1].  This pseudorandom number generator
will use the entropy driver to obtain the seed.

While it uses only 128 bits of state, it's pretty robust for non-crypto
safe applications.

[1] http://vigna.di.unimi.it/xorshift/xoroshiro128plus.c



Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>