mt76: eeprom: add support for big endian eeprom partition

mt76x0e users reported some devices (e.g TP-Link Archer VR200v) have
been flashed with big endian radio partition. Add the possibility to
specify eeprom endianness using big-endian dts property and in case
covert eeprom data in little endian

Tested-by: Kevin Schmidt <kevin.patrick.schmidt@googlemail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>