drm: xlnx: zynqmp_dpsub: Move audio clk from zynqmp_disp to zynqmp_dpsub

The audio clock is an external resource from the DPSUB point of view,
not a resource internal to the display controller. Move it to the
zynqmp_dpsub structure, to allow accessing it from outside the disp
code.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>