MIPS: Fix delay loops which may be removed by GCC.

GCC 4.1 and newer remove empty loops.  This becomes a problem when delay
loops get removed.  Fixed by rewriting to user the proper Linux interface
for such delays.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Reported-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: John Crispin <blogic@openwrt.org>