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/kontron-sl-mx6ul_defconfig b/configs/kontron-sl-mx6ul_defconfig
index efa7a06..f9cebf9 100644
--- a/configs/kontron-sl-mx6ul_defconfig
+++ b/configs/kontron-sl-mx6ul_defconfig
@@ -92,6 +92,7 @@
 CONFIG_MXC_SPI=y
 CONFIG_IMX_THERMAL=y
 CONFIG_USB=y
+CONFIG_USB_MAX_CONTROLLER_COUNT=2
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_MANUFACTURER="FSL"
 CONFIG_USB_GADGET_VENDOR_NUM=0x0525