uboot-mediatek: GatoNetworks GDSP support fixes and updates

uboot-mediatek: GatoNetworks GDSP support fixes and updates

This commit updates the OpenWrt-based U-Boot for this device, allowing
to boot a FIT firmware image stored in an SD card.
The environment has been updated accordingly to allow for managing images
on the two different supported firmware boot media configurations
(NOR or SDMMC).
Erasing the U-Boot environment when upgrading is recommended, so that
the correct DTS overlay is applied.

New behaviour
-------------

The default environment "bootconf" variable is set to
config-1#mt7981b-gatonetworks-gdsp-gps
rendering the GPS module accessible.
You may set it to something like
config-1#mt7981b-gatonetworks-gdsp-sd
to be able to use the MMC controller.
To boot from a firmware image stored in an SD card, you may set the
bootmedia environment variable:
# fw_setenv bootmedia sd
and reboot.
Should the bootmedia variable be absent or it's content not recognized
as valid (e.g., does not contain "nor" or "sd"), NOR boot will be
attempted.

Signed-off-by: Enrico Mioso <[email protected]>