HW SPI: SPI_MODEx

[olikraus]

spi_mode in the info struct should correspond to the Arduino SPI modes SPI_MODE0, SPI_MODE1, etc.
However the numbers 0..3 do not correspond to SPI_MODE0..SPI_MODE3 because internally these macros do have several random values.
As a result, we can not simply pass 0..3 to SPISettings, instead a mapping has to happen first.

More tests are required. At least ST7920 must be tested.

Mode	Clock Polarity (CPOL)	Clock Phase (CPHA)	Output Edge	Data Takover
SPI_MODE0	0	0	Falling	Rising
SPI_MODE1	0	1	Rising	Falling
SPI_MODE2	1	0	Rising	Falling
SPI_MODE3	1	1	Falling	Rising

Image Source: http://www.corelis.com/education/SPI_Tutorial.htm

• Introduce SPI_MODEx mapping in the HW SPI procedures for Arduino
• Check the table above (is this all correct? Does the HW respond correctly?)
• Document the table in the code
• Update the byte procedure (SW SPI)
• Testing

[olikraus]

Verified with ST7920 on Due, Uno and 101
Created a thread in Arduino Forum: http://forum.arduino.cc/index.php?topic=419660.0

[olikraus]

Verified with SH1106 on ESP8266, Due and 101.
Verified with UC1701 on Uno and 101.
Verified with SSD1306 onUno, Due and 101.