test: cmd/fdt: do not assume RNG device exists

test: cmd/fdt: do not assume RNG device exists

In fdt_test_chosen() currently we test if DM_RNG is configured.
CONFIG_DM_RNG=y does not imply that a RNG device actually exists.
For instance QEMU may be called with -device virtio-rng-device or not.
The current test framework evicts the virtio RNG device even if QEMU is
called with -device virtio-rng-device.

In the fdt_test_chosen() check if a RNG device exists.
Ignore 'No RNG device' messages.

Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>