board: sama5d2_xplained: Make SPL work on spiflash

Because before switching to a lower clock source, we must switch
the clock source first instead of last. So before configuring the
PMC_MCKR register, invoke at91_mck_init_down() first.

As said in datasheet, the the size of SPL must not exceed the maximum
size allowed(64Kbytes).

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