Support additional SPI modes

[forGGe]

For now, driver supports only Motorola SPI modes.
TI and Macrowave SPI modes must be supported too.

Listing from the TI driverlib:

//*****************************************************************************
//
// Values that can be passed to SSIConfigSetExpClk.
//
//*****************************************************************************
#define SSI_FRF_MOTO_MODE_0     0x00000000  // Moto fmt, polarity 0, phase 0
#define SSI_FRF_MOTO_MODE_1     0x00000002  // Moto fmt, polarity 0, phase 1
#define SSI_FRF_MOTO_MODE_2     0x00000001  // Moto fmt, polarity 1, phase 0
#define SSI_FRF_MOTO_MODE_3     0x00000003  // Moto fmt, polarity 1, phase 1
#define SSI_FRF_TI              0x00000010  // TI frame format
#define SSI_FRF_NMW             0x00000020  // National MicroWire frame format

Items to cover:

• - implement additional modes support
• - change documentation