uboot-mediatek: add support for GatoNetworks GDSP

uboot-mediatek: add support for GatoNetworks GDSP

The GatoNetworks GDSP is a re-branded version of the R5000 5G Industrial
router from Yinghua Technologies.

Advantages over stock bootloader:
1. supports serving the external GPIO WDT, allowing for easier work in U-Boot
   shell
2. supports cool features like netconsole, easy recovery, scripting and so on
3. allows using FIT image and image integrity validation

and ultimately gives you much more flexibility to implement your tweaks.

Known issues
------------
To make it easier to operate the device, console I/O multiplexing support has
been enabled in U-Boot configuration. Setting I/O related U-Boot environment
variables to something like "serial,nc" will have the desired effect. Still,
setting these variables to such a value in the persistent environment will
lead to a crash and make it impossible to boot the system or recover it. I
decided to leave it on anyway since I think it can be very practical in
development.

Signed-off-by: Enrico Mioso <mrkiko.rs@gmail.com>