board/qemu/aarch64-virt/linux.config: enable VIRTIO_GPU driver

board/qemu/aarch64-virt/linux.config: enable VIRTIO_GPU driver

This driver is intended to be used by mesa virgl Gallium on the guest.
virtio-gpu is enabled by adding "-device virtio-gpu-pci" on the qemu
command line.

It's detected by lspci and dmesg log:

$ lspci
00:01.0 Display controller: Red Hat, Inc. Virtio GPU (rev 01)

$ dmesg
virtio-pci 0000:00:01.0: enabling device (0000 -> 0002)
[drm] pci: virtio-gpu-pci detected at 0000:00:01.0
[drm] virgl 3d acceleration not supported by host
[drm] EDID support available.
[TTM] Zone  kernel: Available graphics memory: 51876 KiB
[TTM] Initializing pool allocator
[TTM] Initializing DMA pool allocator
[drm] number of scanouts: 1
[drm] number of cap sets: 0
[drm] Initialized virtio_gpu 0.1.0 0 for virtio2 on minor 0

The framebuffer interface fb0 is now present in /dev

$ ls /dev/fb*
/dev/fb0

See:
https://www.kraxel.org/blog/2019/09/display-devices-in-qemu/
https://at.projects.genivi.org/wiki/display/WIK4/GENIVI+Technical+Summit+Session+Content+2018?preview=%2F28412356%2F28412481%2F2018-10-11_GeniviBangalorTechSummit_Virtio_GPU.pdf

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>