Generic PDC implementation for SAM3 

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

The implementation is based on the preliminary specifications and the
MSP430 implementations.

- Each PDC supporting user interface (uart, usart, twi, pwm, etc.)
  should wire to the PDC component for the user interface.

- Be sure to enable the peripheral clock for each peripheral's user
  interface before writing to the PDC registers. If the clock is not
  active, some registers will be inactive.
