ARM: atmel: switch to main crystal osc for SPL boot

If without switch to main crystal oscillator, the sama5d3 SoC will
use internal on chip RC oscillator.
In order to get better accuracy, switch to main crystal oscillator.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>