arm: layerscape: Remove CONFIG_USB_MAX_CONTROLLER_COUNT

Because COMFIG_DM_USB has been enabled and will not use it anymore.

Signed-off-by: Ran Wang <ran.wang_1@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: York Sun <york.sun@nxp.com>