ipq806x: Limit NR_CPUS to 2

ipq806x is all dual core processors. ipq807x is quad core. Removes this
from dmesg:

RCU restricting CPUs from NR_CPUS=4 to nr_cpu_ids=2.
RCU: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2

Signed-off-by: Rosen Penev <rosenp@gmail.com>