armsr: enable framebuffer emulation for virtio-gpu/drm displays

armsr: enable framebuffer emulation for virtio-gpu/drm displays

This was discovered when trying to run OpenWrt on Hetzner Cloud's
Arm-based instances.

Hetzner uses QEMU/KVM with virtio-gpu as the main display device,
together with an ACPI firmware. This was not displaying a console
previously.

This setup can be emulated by qemu using options below:
qemu-system-aarch64 \
    -machine virt \
    -bios QEMU_EFI.fd \
    -device virtio-gpu \
    -usb \
    -device qemu-xhci,id=xhci \
    -device usb-tablet,bus=xhci.0 \
    -device usb-kbd,bus=xhci.0 \
    -vnc :0

Signed-off-by: Mathew McBride <matt@traverse.com.au>