Add support for xlnx DRM driver

Hi,

This branch adds support for the xlnx DRM driver used on various Xilinx boards. I've found this support in the Yocto tree of the Xilinx Petalinux distribution here: https://github.com/Xilinx/meta-xilinx/blob/rel-v2020.1/meta-xilinx-bsp/recipes-graphics/xorg-driver/xf86-video-armsoc/0001-src-drmmode_xilinx-Add-the-dumb-gem-support-for-Xili.patch

This patch on top of armsoc 1.4.1 allowed me to bring up Xorg on Ubuntu 20.04 with a ZCU106 board.

Best,
- Loïc Minier