ar71xx: Add "info" partition for TP-Link Archer C7 v5

ar71xx: Add "info" partition for TP-Link Archer C7 v5

This adds the "info" MTD partition, as it is specified in the
ath79 DTS:
https://github.com/openwrt/openwrt/blob/master/target/linux/ath79/dts/qca9563_tplink_archer-c7-v5.dts#L35

This is required to set the WAN MAC address, as it is build based
on the LAN MAC address, which in turn has to be read from the
"info" partition:
https://github.com/openwrt/openwrt/blob/master/target/linux/ath79/dts/qca9563_tplink_archer-x7-v5.dtsi#L184

Backport of commit 9aa8f87d277bf2f666fa5bf90c5efd59dd044d54

Tested-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>