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>

target/linux/ipq806x/config-4.14

@@ -331,7 +331,7 @@ CONFIG_NO_BOOTMEM=y
 CONFIG_NO_HZ=y
 CONFIG_NO_HZ_COMMON=y
 CONFIG_NO_HZ_IDLE=y
-CONFIG_NR_CPUS=4
+CONFIG_NR_CPUS=2
 CONFIG_NVMEM=y
 CONFIG_OF=y
 CONFIG_OF_ADDRESS=y