Refactor to better separate HW and SW SPI

[caternuson]

Current code has a preproc check that will skip all of the code for boards that do not have a hardware SPI capability:

Adafruit_BusIO/Adafruit_SPIDevice.h

Lines 6 to 7 in afbd408

	#if !defined(SPI_INTERFACES_COUNT) || \
	(defined(SPI_INTERFACES_COUNT) && (SPI_INTERFACES_COUNT > 0))

This prevents using even BusIO's software SPI capability in those cases.

Further, there are numerous code segments in the nominal software parts that are relying on being able to import SPI.h. For example, the software constructor is using SPISettings:

Adafruit_BusIO/Adafruit_SPIDevice.cpp

Line 71 in afbd408

_spiSetting = new SPISettings(freq, dataOrder, dataMode);

as well as SPIMode defs.

To better support software SPI only boards, would help to do a general refactor to better separate the HW/SW code.

[caternuson]

Closing. Should be fixed by #108.