dt-bindings: net: wireless: qca,ath9k: convert to the json-schema

This replaces the existing .txt binding file. Furthermore, this
also helps with validating DTS files.

Introduced binding changes:
 1. added inherited mac-address nvmem property
 2. added inherited ieee80211-freq-limit property
 3. added new calibration nvmem property
 4. added second example (taken from the Netgear WNDR3700v2)
    Reason: Setting qca,no-eeprom; takes presedence over
    nvmem-cells. I think a different example is needed,
    because the driver can only reads from one calibration
    source per device.
 5. (re-added) chip list (based on data from ath9k's pci.c)

Added binding .yaml to MAINTAINERS.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210924223509.52525-1-chunkeey@gmail.com