[MIPS] Fix build breakage due to duplicate cpu_clock definition

commit e436d80085133858bf2613a630365e8a0459fd58
Author: Ingo Molnar <mingo@elte.hu>
Date:   Thu Jul 19 21:28:35 2007 +0200

    [PATCH] sched: implement cpu_clock(cpu) high-speed time source

broke the build of several MIPS platforms which were already using the
symbol cpu_clock for the own purposes.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>