ARM: at91: spl: Adjust switching to oscillator for SAMA5D2

As said in 29.5.7 section of SAMA5D2 datasheet, before switching to
the crystal oscillator, a check must be carried out to ensure that
the oscillator is present and that its freqency is valid.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
Reviewed-by: Simon Glass <sjg@chromium.org>