12 bit ADC test program for SAM3S with PDC

@author JeongGil Ko <jgko@cs.jhu.edu>
@author Thomas Schmid <thomas.schmid@utah.edu>

To use PDC with ADC, the sampling rate MUST be controlled by properly
setting the startup, prescal and lowres registers.

Setting the sam3s_adc_channel_config_t config can properly set these
registers.
