avr32: Allow board to define oscillator rates

On our custom board we have other oscillator rates than on atngw100 and
atstk100x.

Currently these rates are hardcoded in arch/avr32/mach-at32ap/at32ap700x.c.

This patch moves them into board specific code.

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