doc: README.xilinx: Add dt bindings

Devicetree bindings are already specified in Linux kernel documentation.
Instead of maintaining same dt bindings in U-Boot doc specify path to dt
bindings in Linux kernel & documenting specific changes that are in U-Boot.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>