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/pico-dwarf-imx7d_defconfig b/configs/pico-dwarf-imx7d_defconfig
index e27b562..3adba80 100644
--- a/configs/pico-dwarf-imx7d_defconfig
+++ b/configs/pico-dwarf-imx7d_defconfig
@@ -87,6 +87,7 @@
 CONFIG_USB=y
 CONFIG_USB_EHCI_HCD=y
 CONFIG_MXC_USB_OTG_HACTIVE=y
+CONFIG_USB_MAX_CONTROLLER_COUNT=2
 CONFIG_USB_GADGET=y
 CONFIG_USB_GADGET_MANUFACTURER="FSL"
 CONFIG_USB_GADGET_VENDOR_NUM=0x0525