MIPS: Include linux/cpu.h for arch_cpu_idle()

MIPS: Include linux/cpu.h for arch_cpu_idle()

arch/mips/kernel/idle.c provides our implementation of the
arch_cpu_idle() function, but doesn't include the linux/cpu.h header
which declares it. This leads to a warning from sparse:

  arch/mips/kernel/idle.c:256:6: warning: symbol 'arch_cpu_idle' was not
    declared. Should it be static?

Fix this by including linux/cpu.h to get the declaration of
arch_cpu_idle().

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: trivial@kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17169/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>