Fix warnings from import of libfdt

cmd_fdt.c: In function fdt_print:
cmd_fdt.c:586: warning: assignment discards qualifiers from pointer target type
cmd_fdt.c:613: warning: assignment discards qualifiers from pointer target type
cmd_fdt.c:635: warning: assignment discards qualifiers from pointer target type
cmd_fdt.c:636: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>