Updated SPI interfaces.
Removed CS pin configuration from common SPI driver.
All pins made optional to support various cases: SCK pin left
unused in 1-Wire emulation, MISO is unavailable for some TFT display,
MOSI makes no sense when there is no data input on slave device.
Minor updates in descriptions.
Added initial version of OneWire over SSP.
SSP is configured in 16-bit mode.
MISO and MOSI pins are connected together, SCK and SSEL unused.