samples: counter/alarm: add support for Atmel SAM boards

Add support for sam_e70_xplained, sam_v71_xult based on TC0 counter.

Note: TC module is a 16-bit counter. Even with slow, 32768 Hz input
clock the time span counted by the driver is short.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>