sam3x support.
sam3x: clean up gpio mess.
sam3x: Add some PMC convenience functions.
sam3x: Add USART registers and come convenience functions.
sam3x: Add gpio_init convenience function.
sam3x: Add some pwm convenience funcitons.
sam3x: Correct pmc_xtal_enable to use startup time parameter.
sam3x: Add missing return types to pwm functions.
sam3x: Add convenince functions for setting clocks and usart baudrate.
sam3x: Add usart funciton to enable rx interrupt.