Software SPI allows non-standard SPI pins to be used for interfacing with SPI devices

Dependents:   ADNS5090 PB_Emma_Ethernet spi_test Final_Project ... more

Made MISO pin optional by accepting "NC" pin name

Some devices (like displays) do not have a MISO pin. The hardware SPI class allows passing NC for the MISO pin name, but SWSPI does not (because DigitalIn does not accept NC.) This PR allows passing NC for MISO in the constructor.

Software SPI allows non-standard SPI pins to be used for interfacing with SPI devices SPI