dm: move platform data headers to include/dm/platform_data

dm: move platform data headers to include/dm/platform_data

The platform_data definitions are generally referenced from both
drivers and board files.  That is why header files defining
platform_data sturectures are placed in "include" directory,
but our top level "include" directory is already too cluttered.

Let's collect platform_data definitions under the directory
"include/dm/platform_data" like Linux gathers ones around under
"include/linux/platform_data".

This commit moves two header files:

  include/serial_mxc.h -> include/dm/platform_data/serial_mxc.h
  include/serial_pl01x.h -> include/dm/platform_data/serial_pl01x.h

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>