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>
diff --git a/include/configs/sama5d2_xplained.h b/include/configs/sama5d2_xplained.h
index 891218d..aedd568 100644
--- a/include/configs/sama5d2_xplained.h
+++ b/include/configs/sama5d2_xplained.h
@@ -61,7 +61,7 @@
 /* SPL */
 #define CONFIG_SPL_FRAMEWORK
 #define CONFIG_SPL_TEXT_BASE		0x200000
-#define CONFIG_SPL_MAX_SIZE		0x18000
+#define CONFIG_SPL_MAX_SIZE		0x10000
 #define CONFIG_SPL_BSS_START_ADDR	0x20000000
 #define CONFIG_SPL_BSS_MAX_SIZE		0x80000
 #define CONFIG_SYS_SPL_MALLOC_START	0x20080000