ARM64: zynqmp: Add description for LPDDMA channel usage

LPDDMA default allows only secured access.
inorder to enable these dma channels,
one should ensure that it allows non secure access.
This patch updates the same.

Reported-by: Sai Pavan Boddu <saipava@xilinx.com>
Signed-off-by: Kedareswara rao Appana <appanad@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>