Add virtual provider for dtb. This class can be used to generate dtb
from static devicetree, for reference see meta-xilinx layer
meta-xilinx/meta-xilinx-bsp/recipes-bsp/device-tree/device-tree.bb
By providing a virtual provider we can use it to differentiate between
in-kernel dtb or an external one. This can be set in local.conf as
PREFERRED_PROVIDER_virtual/dtb = "devicetree"
Signed-off-by: Manjukumar Matha <manjukumar.harthikote-matha@xilinx.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
