Add LEDs to the line card

[mjgerm]

LEDs could be used for activity/link lights, vlan indicators, service/patch panel indicators, and more. Adding a row of 8x RGB LEDs to the bottom of the line card

The SX1509 chip has LED drivers, as discussed, and supports blinking/breathing effects.

The Wurth Electronics 155124M173200 (Digikey 732-11414-1-ND) is a good side-mounted LED, which could be placed right on the edge of the board, mitigating the need for light-pipes.

[mjgerm]

For 8x RGB LEDs, we need 24 pins, which means at least 2 16x GPIO expanders.
Therefore, there's 8 spares - might want to attach these to the PHYs RESET pins.

Note that not all IO pins on the SX1509 are created equal, only half support the "breathing" effect. If we want to support the breathing effect on all LEDs, we need 3 per card, and should probably share OSCIO/NRESET between them & between cards, to sync LED effects. (not strictly necessary, but it would be neat)