Seeed Xiao BLE boards: enable 2nd LED

[mestrode]

The Seeed Xiao Board has a small tripple LED w/ RGB colors.
The old code contained a three year old commend, saying

// The board has 3 leds, but changing the number here causes OTA issues.

Since other boards obviously don't have any problems, guess the comment is simply outdated. However if there were problems, say should be covered after cleanup of the button system (#383).

Unfortunately I'm not able to compile and test this PR on my own.

• Test on board required, since this are no separated LED, but a combined part on PCB.
• May a quick test, OTA is not effected by this change (disprove old code commend)

[mestrode]

Source for Led pins:

• Seeedstudio Xiao BLE FAQ
• Xiao BLE Schematic last page, bottom left;

Since the Xiao BLE version without SENSE is just a subset of mounted parts, the same schematic applies.

[hathach]

lgtm, thank you