arch: arm: dts: fsl-lx2160a.dtsi: tag serial nodes with bootph-all

Tag the serial nodes with bootph-all in order to have these nodes and
the drivers available before relocation.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>