serial: ucc_uart: explicitly include soc/fsl/cpm.h

This driver uses #defines from soc/fsl/cpm.h, so instead of relying on
some other header pulling that in, do that explicitly. This is
preparation for allowing this driver to build on ARM.

Reviewed-by: Timur Tabi <timur@kernel.org>
Acked-by: Timur Tabi <timur@kernel.org>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Li Yang <leoyang.li@nxp.com>