tty: serial: imx: add dependence and build for earlycon

Add the earlycon dependence and add earlycon Makefile support
to allow to build the driver.

Fixes: 699cc4df ("tty: serial: imx: add imx earlycon driver")
Signed-off-by: Fugang Duan <fugang.duan@nxp.com>
Link: https://lore.kernel.org/r/20200810060652.3436-1-fugang.duan@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>