Add IMX3112 mux and APML devices to Grapefruit

[mkeeter]

TODO

• Test on actual hardware (standish)

[mkeeter]

This is currently failing on real hardware (Grapefruit + Ruby). We can talk to the mux and read back its version / ID / etc, but once we select mux channel 0 (0x40), we don't actually see anything talking back to us on that bus (other than the mux itself).

It's unclear whether this is a mux issue or whether our SP5 isn't configured for SB-TSI; @nathanaelhuffman is going to investigate the feasibility of soldering to SDA / SCL after the mux to narrow it down.