Remaining undeclared portInputRegister and digitalPinToPort

[jgfoster]

This is a follow-up to #193. With that issue we have these identifiers declared for some platforms but there are others that do not yet work. To see this go to Arduino-CI/Adafruit_BusIO and remove (or rename) .arduino-ci.yml and run ./scripts/test.sh and you will see things like the following:

/Users/jfoster/Documents/Arduino/libraries/Adafruit_BusIO/src/Adafruit_SPIDevice.cpp:58:33: error: use of undeclared identifier 'portInputRegister'
    misoPort = (BusIO_PortReg *)portInputRegister(digitalPinToPort(misopin));

This issue is low priority (to the author!), but represents an issue that might arise for others so seems to warrant a record here.