Add support for building i.MX serial driver as module.

The changes of the patch:
- imx console driver can be built as module.
- move out earlycon code to separated driver like imx_earlycon.c,
  and imx earlycon driver only support build-in.
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Link: https://lore.kernel.org/r/20200724070815.11445-2-fugang.duan@nxp.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>