dt-bindings: mt76: support pointing to EEPROM using NVMEM cell

All kind of calibration data should be described as NVMEM cells of NVMEM
devices. That is more generic solution than "mediatek,mtd-eeprom" which
is MTD specific.

Add support for EEPROM NVMEM cells and deprecate existing MTD-based
property.

Cc: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>