Add missing macro `portInputRegister`

[ianfixes]

Issue Summary

portOutputRegister isn't defined, will need to define it in a similar style to portOutputRegister(port) in Godmode.h.

#define portInputRegister(P) ( (volatile uint8_t *)( pgm_read_word( port_to_input_PGM + (P))) )

It may be AVR-only:
https://garretlab.web.fc2.com/en/arduino/inside/hardware/arduino/avr/cores/arduino/Arduino.h/portInputRegister.html

Found in https://github.com/Open-Acidification/TankControllerLib/pull/11/checks?check_run_id=1398445342#step:6:31 as part of Open-Acidification/TankControllerLib#11