[AVR32] Don't enable clocks with no users

Bring the code that sets the initial PM clock masks in line with the
comment preceding it by only enabling clocks that have users != 0.
Fix SM clock definition and avr32_hpt_init() so that the SM and TC0
clocks keep ticking.

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