MIPS: BCM47XX: do not use cpu_wait instruction on BCM4706

The BCM4706 has a problem with the CPU wait instruction. When r4k_wait
or r4k_wait_irqoff is used will just hang and not return from a
msleep(). Removing the cpu_wait functionality is a workaround for this
problem. The BCM4716 does not have this problem.

The BCM4706 SoC uses a MIPS 74K V4.9 CPU.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/6288/