avr32: Rename pm_init() as clk_init() and make SoC-specific

avr32: Rename pm_init() as clk_init() and make SoC-specific

pm_init() was always more about clock initialization than anything
else. Dealing with PLLs, clock gating and such is also inherently
SoC-specific, so move it into a SoC-specific directory.

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