ipq4018: add TCSR definitions from Linux.

These are hardware configuration options which are required in
the linux/openwrt device trees for the IPQ4018/IPQ4019 devices.

Since this isn't obtained from linux upstream but instead from
openwrt, this can't go in contrib; instead it is going in
sys/dts/include/ .

Obtained from: OpenWRT

Tested:

• IPQ4019 ASUS RT-AC58U AP, initial bootstrapping