[AVR32] Split SM device into PM, RTC, WDT and EIC

Split the SM platform device into separate platform devices for PM,
RTC, WDT and EIC. This is more correct according to the documentation
and allows us to simplify the code a little.

Also turn the EIC driver into a real platform driver.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
Acked-by: Hans-Christian Egtvedt <hcegtvedt@atmel.com>