ath79: add ath9k calibration data MAC addresses patching

ath79: add ath9k calibration data MAC addresses patching

This patch copies over the MAC patching helper functions from lantiq's
target/linux/lantiq/base-files/etc/hotplug.d/firmware/12-ath9k-eeprom
file.

Not all vendors bothered to write the correct MAC addresses for the
ath9k wifi into the calibration data. And while ath9k does have some
special dt-properties to extract the addresses from a fixed position,
there are still devices that require userspace to edit or modify
the caldata.

In my case, the MAC address for the Wi-Fi device is stored in an
unsorted key-value based "nvram" database and there's an existing
userspace tool to extract the data.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>