[AVR32] Clean up cpu identification and add features bitmap

Clean up the cpu identification code, using definitions from
<asm/sysreg.h> instead of hardcoded constants. Also, add a features
bitmap to struct avr32_cpuinfo to allow other code to make decisions
based upon what the running cpu is actually capable of.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>