spi: Rework the SPI interface

spi: Rework the SPI interface

Previous SPI API required the caller to know the SPI interface and index
of the device on this interface. This was because the interface data
structure had links to devices. Now, this is reversed - an SPI device
has a link to SPI interface it is connected to. Therefore, the user of
SPI API needs to specify only the device to communicate with. The SPI
interface is determined from the device internally.

Also some refactoring was done. Things than does not need to be public
were moved from .h to .c and declarations needed in spi.h were moved
there from spi_tms570.h.