sandbox: Add an SPL loader for UPL

Add support for loading a UPL image from SPL. This uses the simple FIT
implementation, but also loads the full FIT just to permit more testing.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/include/asm/spl.h b/arch/sandbox/include/asm/spl.h
index d50d9ad..d824b21 100644
--- a/arch/sandbox/include/asm/spl.h
+++ b/arch/sandbox/include/asm/spl.h
@@ -18,6 +18,7 @@
 	BOOT_DEVICE_NOR,
 	BOOT_DEVICE_SPI,
 	BOOT_DEVICE_NAND,
+	BOOT_DEVICE_UPL,
 };
 
 /**