subsys: random: xoroshiro128: Use SYS_INIT hook to initialize properly

Not only removes a branch during normal operation, but also ensures
that, by initializing at the PRE_KERNEL_2 stage, and granting
privileges to all threads to the semaphore, this code will work in
early boot situations and in user mode.

This assumes that entropy drivers will all initialize during
PRE_KERNEL_1 stage.  All in-tree drivers do that.

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