tty: serial: imx: enable earlycon by default if IMX_SERIAL_CONSOLE is enabled

Since 699cc4df (tty: serial: imx: add imx earlycon driver), the earlycon
part of imx serial is a separate driver and isn't necessarily enabled anymore
when the console is enabled. This causes users to loose the earlycon
functionality when upgrading their kenrel configuration via oldconfig.

Enable earlycon by default when IMX_SERIAL_CONSOLE is enabled.

Fixes: 699cc4df

 (tty: serial: imx: add imx earlycon driver)
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: https://lore.kernel.org/r/20201105204026.1818219-1-l.stach@pengutronix.de
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>