subsys/random: sys_rand32_get() implementation that uses entropy API

Instead of every hardware number generator driver providing an
implementation of this function, use the random device API to
centralize the implementation of this function.

This is a very simplistic function that can be seen as a stepping stone
to refactor the random number generation in Zephyr.

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