mips: mt76xx: gardena-smart-gateway: Add factory data variable handling

mips: mt76xx: gardena-smart-gateway: Add factory data variable handling

Some factory data is stored in the SPI NOR and needs to get extracted
from there into U-Boot environment variables.

This patch also includes a board-specific command "fd_write" to
provide some dummy / default values for this factory-data in the SPI
NOR flash. This should only be necessary for testing purposes though.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>