riscv: qemu: imply CONFIG_DM_RNG

The EFI_RNG_PROTOCOL is needed for Linux' KASLR.

QEMU can provide a virtio-rng device to emulate a hardware random number
generator which is supported by our virtio_rng driver.

Enabling CONFIG_DM_RNG will enable CONFIG_VIRTIO_RNG and
CONFIG_EFI_RNG_PROTOCOL by default too.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>