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-c7-v5.dts#L35

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