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/ls1021aqds_nor_defconfig b/configs/ls1021aqds_nor_defconfig
index cf4f79d..426a4be 100644
--- a/configs/ls1021aqds_nor_defconfig
+++ b/configs/ls1021aqds_nor_defconfig
@@ -28,3 +28,4 @@
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y
 CONFIG_USB_XHCI_HCD=y
+CONFIG_USB_XHCI_DWC3=y