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/am57xx_evm_nodt_defconfig b/configs/am57xx_evm_nodt_defconfig
index 373aa9e..1cf82d2 100644
--- a/configs/am57xx_evm_nodt_defconfig
+++ b/configs/am57xx_evm_nodt_defconfig
@@ -24,4 +24,5 @@
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y
 CONFIG_USB_XHCI_HCD=y
+CONFIG_USB_XHCI_DWC3=y
 CONFIG_OF_LIBFDT=y