mt76: support reading EEPROM data embedded in fdt

Some platforms boot from SD card and don't come with calibration data
stored anywhere on the board.
As EEPROM data is rather small it can be embedded into the device tree
to be loaded from there by the mt76.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>