The devicetrees for various platforms already exceed 16k. Add a define
CONFIG_SYS_FDT_SIZE to specify the FDT size, and set to 16k for the
two boards that define this CONFIG_SYS_FDT_BASE parameter. This
allows platforms with larger devicetree blobs to boot from NOR.
Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>