usb: move CONFIG_USB_XHCI_DWC3 to Kconfig

Create an entry for "config USB_XHCI_DWC3" in Kconfig and
switch over to it for all boards.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/configs/smdk5420_defconfig b/configs/smdk5420_defconfig
index ad599fc..3bda611 100644
--- a/configs/smdk5420_defconfig
+++ b/configs/smdk5420_defconfig
@@ -37,4 +37,5 @@
 CONFIG_USB=y
 CONFIG_DM_USB=y
 CONFIG_USB_XHCI_HCD=y
+CONFIG_USB_XHCI_DWC3=y
 CONFIG_VIDEO_BRIDGE=y