arm: atmel: sama5d3: add spi spl boot support

Add SPI SPL boot support for sama5d3xek board.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
diff --git a/arch/arm/cpu/at91-common/spl.c b/arch/arm/cpu/at91-common/spl.c
index 37c0cc4..2598e09 100644
--- a/arch/arm/cpu/at91-common/spl.c
+++ b/arch/arm/cpu/at91-common/spl.c
@@ -52,6 +52,8 @@
 {
 #ifdef CONFIG_SYS_USE_MMC
 	return BOOT_DEVICE_MMC1;
+#elif CONFIG_SYS_USE_SERIALFLASH
+	return BOOT_DEVICE_SPI;
 #endif
 	return BOOT_DEVICE_NONE;
 }