Fix indent and small cosmetic changes
[sam] SPI library: added SPI_HAS_EXTENDED_CS_PIN_HANDLING flag

This allows to check if SPI provides extended API to allow automatic
CS pin handling.
[sam] Simplified formula for SPI speed
[sam] Implementation of SPI multibyte transfer
Merge pull request #2299 from cmaglie/ide-1.5.x-spi-updates

Missing SPI Transaction API methods on Due (SAM3X)