microblaze: Remove _fdt_start casts

_fdt_start is an array, i.e. a pointer.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: microblaze-uclinux@itee.uq.edu.au
Signed-off-by: Michal Simek <michal.simek@xilinx.com>