Allow SPI to work without a CS pin

[jfpoilpret]

In many boards, one may use a single SPI device for which there is no need for a specific CS "Chip Select" pin, as this could be directly forced on the device to the proper level (generally GND).

Currently, FastArduino SPIDevice class, the base of all SPI devices, requires the following template arguments:

• board::DigitalPin CS
• ChipSelect CS_MODE

FastArduino SPIDevice class should make these arguments optional and perform CS pin management only when one CS pin is provided.

One easy way to handle this improvement, without a deep redesign of SPIDevice class and all current subclasses implemented in FastArduino or outside, would be to:

• use the "fake" pin NONE in enum DigitalPin: using that pin would mean "no pin"
• in SPIDevice, perform CS management only when CS pin is a real digital pin

In the best case, SPI would not need to be changed as gpio namespace already handles NONE pins by providing empty methods for everything.

[jfpoilpret]

As expected initially, FastArduino SPIDevice can work out of the box without the need of a CS pin. You just pass DigitalPin::NONE for CS and that's it! You should then be able to handle a single SPI device, provided its CS pin is forced to GND.

However, do perform some experiments to ensure your SPI device does support this way of working, with an always-0 CS pin. It seems some devices do not work properly with such wiring!

I will close this issue as there is nothing to do, besides updating SPIDevice API doc a little bit.

[jfpoilpret]

No need for a fix. Doc updated on master in commit #f3a571d