Function generator code modified to work on the Freescale KL25Z

Dependencies:   mbed

Fork of AD9850 function generator SPI driver by Liam G

This is an update to the original AD9850 SPI driver produced by Liam Goudge.

The main changes are to assign the appropriate pins and to send the 40-bit AD9850 programming word as five 8-bit packets. This is necessary because the KL25Z only supports 8-bit SPI transfers.

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