fdt: Correct cast for sandbox in fdtdec_setup_memory_size()

This gives a warning with some native compilers:

lib/fdtdec.c:1203:8: warning: format ‘%llx’ expects argument of type
   ‘long long unsigned int’, but argument 3 has type
   ‘long unsigned int’ [-Wformat=]

Fix it with a cast.

Signed-off-by: Simon Glass <sjg@chromium.org>