v4l: xilinx: scd: Allocate all channels in one go

v4l: xilinx: scd: Allocate all channels in one go

This eases memory management and channels initialization, we don't have
to iterate over subnodes anymore.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Satish Kumar Nagireddy <satish.nagireddy.nagireddy@xilinx.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>