clk: zynqmp: Add support for for DP audio/video clocks

Add support for getting rate for DP audio and video clocks.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/8792efe1fd9715f7c8a2e1e24f0454fb5b25d833.1648552434.git.michal.simek@xilinx.com