x86/64: enable MMIO_CMDLINE_DEVICES for firecracker support

x86/64: enable MMIO_CMDLINE_DEVICES for firecracker support

This Kernel option allows to run OpenWrt witin a `firecracker` micro VM.

Firecracker is a KVM-based tool for superfast booting VMs on x86_64 and
aarch64. It makes rootfs available to the guest as a virtio-mmio device
and passes its address via the kernel cmdline. A kernel without
CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES will not recognize the rootfs
virtio-mmio device.

Suggested-by: Packet Please <pktpls@systemli.org>
Signed-off-by: Paul Spooren <mail@aparcar.org>