usb: Introduce CONFIG_SPL_DM_USB

This allows building the SPL without driver model for USB. Since
CONFIG_SPL_DM_USB is enabled if and only if CONFIG_DM_USB was enabled
before, this patch does not change the build behaviour.

Signed-off-by: Sven Schwermer <sven@svenschwermer.de>
diff --git a/drivers/usb/common/Makefile b/drivers/usb/common/Makefile
index 55e0547..3bedbf2 100644
--- a/drivers/usb/common/Makefile
+++ b/drivers/usb/common/Makefile
@@ -3,6 +3,6 @@
 # (C) Copyright 2016 Freescale Semiconductor, Inc.
 #
 
-obj-$(CONFIG_DM_USB) += common.o
+obj-$(CONFIG_$(SPL_)DM_USB) += common.o
 obj-$(CONFIG_USB_EHCI_FSL) += fsl-dt-fixup.o fsl-errata.o
 obj-$(CONFIG_USB_XHCI_FSL) += fsl-dt-fixup.o fsl-errata.o