microblaze: Add FDT support

This patch adds FDT (flattened device tree) support to microblaze arch.

Tested with Linux arch/microblaze kernels with and without compiled in
FDT on Xilinx ML506 board.

Signed-off-by: Arun Bhanu <arun@bhanu.net>
Signed-off-by: Michal Simek <monstr@monstr.eu>