parallella-hdmi: Refactored and updated device tree

* Refactored the device tree to re-use the base parallella.dtsi and
  seperate the hdmi segment into parallella-hdmi.dtsi
* Fixed bindings for vdma (added interrupt-parent)
* With the use of the meta-xilinx zynq7-base.dtsi the i2c is now using
  the correct cadence driver binding for newer kernels.
linux-parallella: Added missing configs for HDMI

* Added missing CONFIG_COMMON_CLK_AXI_CLKGEN for the 'adi,axi-clkgen'
  device
* Added CONFIG_XILINX_DMA for the axi-vdma driver
linux-parallella: Add kernel device tree building

* Additional to the device trees provided in the layer, build the device
  trees for the parallella from the kernel source
Merge pull request #2 from nathanrossi/fix-hdmi-video-and-refactor

Fix HDMI Video and refactor device trees