Doc: usb: chipidea: need to build both kernel Image and modules

When tried to enable OTG FSM, we need to rebuild both kernel Image
and modules, since there are some codes at gadget modules which are
controlled by related configurations.

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>