rng: detect RISC-V Zkr RNG device in bind method

The existence of devices should be checked in the bind method and not in
the probe method. Adjust the RISC-V Zkr RNG driver accordingly.

Use ENOENT (and not ENODEV) to signal that the device is not available.

Fixes: ceec977b ("rng: Provide a RNG based on the RISC-V Zkr ISA extension")
Reported-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>