brcm63xx: sync mips counters during bringup of cpus

Fixes gettimeofday returning different values depending on which cpu
it is called.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40387 3c298f89-4303-0410-b956-a3cf2f4a3e73