dt-bindings: serial: fsl-lpuart: add i.MX7ULP support

The lpuart of imx7ulp is basically the same as ls1021a. It's also
32 bit width register, but unlike ls1021a, it's little endian.
Besides that, imx7ulp lpuart has a minor different register layout
from ls1021a.

Cc: devicetree@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Stefan Agner <stefan@agner.ch>
Cc: Mingkai Hu <Mingkai.Hu@nxp.com>
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>