board/khadas/vim3: add the path of the devicetree in extlinux.conf

Without this patch, the devicetree used by linux at runtime is
the u_boot devicetree.

While using rusticl on the khadas vim3 board with the default
extlinux.conf, mesa's panfrost driver return "gpu shed timeout"
errors. Adding "devicetree /meson-g12b-khadas-vim3.dtb" to the
extlinux.conf file solve the problem. Linux successfully boot
with and without the change but without this patch the devicetree
used by linux at runtime is the u-boot devicetree.

The differences between the u-boot devicetree and the linux
devicetree are:
-The u-boot devicetree contains two more nodes than the linux devicetree. Those nodes are:
    - smbios : compatible option is set to "u-boot,sysinfo-smbios".
    - __symbols__ : present if -@ or --symbols is used during the devicetree compilation.
-Most of u-boot devicetree nodes have a "phandle" property.
-SoC and vpu have a "u-boot,dm-pre-reloc;" property.
-Some aliases are different.
-The u-boot operating points tables contains more nodes.
The size difference between the u-boot devicetree and the linux
devicetree is approximately 3.5kB.

During runtime, the linux devicetree have the "phandle" and
"u-boot,dm-pre-reloc;" properties and the smbios node. So,
The runtime devicetree is the one from u-boot.

Signed-off-by: Antoine Coutant <antoine.coutant@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

board/khadas/vim3/extlinux.conf

@@ -1,4 +1,5 @@
 default buildroot
 label buildroot
    kernel /Image
+   devicetree /meson-g12b-a311d-khadas-vim3.dtb
 append root=/dev/mmcblk0p2 rootwait console=ttyAML0,115200