mips: dts: ralink: mt7621: add GB-PC2 LEDs

Add the missing LEDs for GB-PC2. The ethblack-green, ethblue-green, power
and system LEDs weren't added previously, because they don't exist on the
device schematics. Tests on a GB-PC2 by me and Petr proved otherwise.

The i2c bus cannot be used on GB-PC2 as its pins are wired to LEDs instead,
and GB-PC1 does not use it. Therefore, do not enable it on both devices.

Link: https://github.com/ngiger/GnuBee_Docs/blob/master/GB-PCx/Documents/GB-PC2_V1.1_schematic.pdf


Tested-by: Petr Louda <petr.louda@outlook.cz>
Signed-off-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>