Convert CONFIG_USB_MAX_CONTROLLER_COUNT to Kconfig

This converts the following to Kconfig:
   CONFIG_USB_MAX_CONTROLLER_COUNT

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/variscite_dart6ul_defconfig b/configs/variscite_dart6ul_defconfig
index f19dcbd..0f9cc8d 100644
--- a/configs/variscite_dart6ul_defconfig
+++ b/configs/variscite_dart6ul_defconfig
@@ -58,6 +58,7 @@
 CONFIG_MXC_UART=y
 CONFIG_IMX_THERMAL=y
 CONFIG_USB=y
+CONFIG_USB_MAX_CONTROLLER_COUNT=2
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_MANUFACTURER="Variscite"
 CONFIG_USB_GADGET_VENDOR_NUM=0x0525