dm: arm: rpi: Move to driver model for USB

Start using driver model for USB on the Raspberry Pi. The dwc2 supports
this now so this is just a config change.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/rpi_2_defconfig b/configs/rpi_2_defconfig
index 9875f5d..9b7fee7 100644
--- a/configs/rpi_2_defconfig
+++ b/configs/rpi_2_defconfig
@@ -17,6 +17,7 @@
 CONFIG_MMC_SDHCI=y
 CONFIG_MMC_SDHCI_BCM2835=y
 CONFIG_USB=y
+CONFIG_DM_USB=y
 CONFIG_USB_STORAGE=y
 CONFIG_USB_KEYBOARD=y
 CONFIG_SYS_WHITE_ON_BLACK=y