GEOM: Add fdt_slicer to the GEOM flashmap module for fdt-based platforms

geom_flashmap depends on a slicer being available in order to do any
work. On fdt platforms this is provided by fdt_slicer, but this needs
to be available. Often it's compiled into the kernel for platforms that
boot from the relevant media, but this is not always the case. Add the
file to the geom_flashmap module so that it can be used on platforms
which don't always need this functionality available.