ramips: improve YunCore AX820 LEDs

ramips: improve YunCore AX820 LEDs

At least two AX820 hardware variants are known to exist, but they cannot
be distinguished (same hardware revision, no specific markings).

They appear to have the same LED hardware, but wired differently:

- One has a red system LED at GPIO 15, a green wlan2g LED at GPIO 14 and
  a blue wlan5g LED at GPIO 16;
- The other only offers a green system LED at GPIO 15, with GPIO 14 and
  16 being apparently not connected

Finally, a Yuncore datasheet says the canonical wiring should be:
- Blue wlan2g GPIO 14, green system GPIO 15, red wlan5g GPIO 16

All GPIOs are tied to a single RGB LED which is exposed via lightpipe on
the device front casing.

Considering the above, this patch exposes all three LEDs, preserves the
common system LED (GPIO 15) as the openwrt status LED, and removes the
color information from the LEDs names since it is not consistent across
hardware. The LED naming is made consistent with other YunCore devices.
A note is added in DTS to ensure this information is always available
and prevent unwanted changes in the future.

Fixes: #10131 "YunCore AX820: GPIO LED not correct"
Reviewed-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>