Second buffered UART pinout for MEGAphone Prototyping

[gardners]

The MEGAphone requires two UARTs: One to talk to the cellular modem, and the other to talk to the power control FPGA module.

For the cellular modem, we can use the same UART pins as provisioned for the "accessory" on the MEGA65 Expansion Board (see 768-* branch) which normally hosts an ESP32 WiFi module, but can just as easily host a cellular modem.

For the power control FPGA we need a second UART with 3.3V signalling. Here our options are more constrained, especially if we want it to work on revisions from the R3 on-wards.

The easiest option here is to re-use the currently ignored JTAG pins for the keyboard on the keyboard connector. On the R3 those go via the MAX10, but are basically passed through in normal operating mode. On the R4+, they are directly connected to the main FPGA. Using a split cable we can put a UART on TDO (RX to MEGA65) and TDI (TX from MEGA65). As TCK is not ticking this is safe, as the worst that could happen is the keyboard FPGA drives TDO low or high, in which case nothing happens on this UART except one junk character received by the MEGA65. TDI flapping with UART traffic while TCK is not ticking will have no practical effect. When the split cable is attached, then the UART can obviously operate freely.