am33xx: Add SPI SPL as an option

Add the required config.mk logic for this SoC as well as the BOOT_DEVICE
define.  Finally, enable the options on the am335x_evm.

Signed-off-by: Tom Rini <trini@ti.com>
diff --git a/arch/arm/cpu/armv7/am33xx/config.mk b/arch/arm/cpu/armv7/am33xx/config.mk
index 5750bbd..babf0eb 100644
--- a/arch/arm/cpu/armv7/am33xx/config.mk
+++ b/arch/arm/cpu/armv7/am33xx/config.mk
@@ -13,6 +13,7 @@
 #
 ifdef CONFIG_SPL_BUILD
 ALL-y	+= $(OBJTREE)/MLO
+ALL-$(CONFIG_SPL_SPI_SUPPORT) += $(OBJTREE)/MLO.byteswap
 else
 ALL-y	+= $(obj)u-boot.img
 endif