dtc: Simplify asm_emit_string() implementation

Using %.*s format helps making asm_emit_string() not modify its "str"
parameter.

While at it, constify the "str" parameter of bin_emit_string() and
asm_emit_string(), as these function no longer modify it.

Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>