random: Introduce random device API.

Introduce random device API analgous to other device driver classes in
Zephyr.  Modify the the KSDK random driver to implement the API.

We retain the sys_rand32_get() interface for now on the assumption it
will eventually relocate to or be otherwise replaced by an entropy
management system.

The existing TEST_RANDOM_GENERATOR related drivers that do not
generate entropy are not modified to expose this driver API since they
cannot generate entropy.

Change-Id: I60b2d5afddf242e802a1d9014c99579870fb7472
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>