Ide 1.5.x spi 16bit transfer

[MadVikingGod]

Added functions to the Due SPI library to enable 16bit SPI transfers.

To use:

• Initialize as normal, then use SPI.setTransferWidth(pin, width).

void setup() {
    SPI.begin(4);
    SPI.setTransferWidth(4,SPI_WIDTH_16);
}

• Use SPI.transfer() as normal, but can use upto 16 bits.

SPI.transfer(4,0xAA55);

The LSBFIRST setting only works for 8 bits.
TODO: twiddle 9-16 bit words to be reversed for LSB transfers. This is made more difficult because of the right-justified contents of the transfer data format.

-MVG

[cmaglie]

@MadVikingGod
thanks for the patch but we are not going to merge this one because it's very SAM3X specific.
Implementing a function like SPI.transfer16(..) is more suited for this purpose, see #3961