virtio: VIRTIO_RNG depends on DM_RNG

Add the missing Kconfig dependency and let VIRTIO_RNG default to yes.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>