xlnx_dp: Introduce a vblank signal

xlnx_dp: Introduce a vblank signal

Add a periodic timer which raises vblank at a frequency of 30Hz.

Note that this is a migration compatibility break for the
xlnx-zcu102 board type.

Signed-off-by: Sai Pavan Boddu <saipava@xilinx.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Frederic Konrad <fkonrad@amd.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20220601172353.3220232-3-fkonrad@xilinx.com
Changes by fkonrad:
  - Switched to transaction-based ptimer API.
  - Added the DP_INT_VBLNK_START macro.
Signed-off-by: Frederic Konrad <fkonrad@amd.com>
[PMM: bump vmstate version, add commit message note about
 compat break]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>