[AVR32] SMC configuration in clock cycles

This patch makes the SMC configuration take timings in clock cycles
instead of nanoseconds. A function to calculate timings in clock
cycles is added.

This patch removes the rounding troubles of the previous SMC
configuration method.

[hskinnemoen@atmel.com: fix atstk1002/atngw100 flash config]
Signed-off-by: Kristoffer Nyborg Gregertsen <gregerts@stud.ntnu.no>
Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>