rng: fix ARMv8.5 RNDR driver

In different parts of our code we assume that the first RNG device is the
one to be used. Therefore it is preferable to detect the availability of
the RNDR register already in the bind method.

For signaling the non-existence of a device the driver model requires using
ENOENT (and not ENODEV).

Fixes: 31565bb0 ("driver: rng: Add DM_RNG interface for ARMv8.5 RNDR registers")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Andre Przywara <andre.przywara@arm.com>