avr32: Replace static clock list with dynamic linked list

This replaces the at32_clock_list array with a linked list.
Clocks can now be registered (added) to the list.

Signed-off-by: Alex Raimondi <raimondi@miromico.ch>
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>