vbe: Support reading the next SPL phase via VBE

Add an SPL loader to obtain the next-phase binary from a FIT provided
by the VBE driver.

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 312aef7..2f8b5fc 100644
--- a/arch/sandbox/include/asm/spl.h
+++ b/arch/sandbox/include/asm/spl.h
@@ -11,6 +11,7 @@
 	BOOT_DEVICE_MMC2,
 	BOOT_DEVICE_MMC2_2,
 	BOOT_DEVICE_BOARD,
+	BOOT_DEVICE_VBE,
 };
 
 /**