fdt_support: define stub for fdt_fixup_mtdparts

Define an inline stub for fdt_fixup_mtdparts in the case that
CONFIG_FDT_FIXUP_PARTITIONS is not defined. This avoids the need
to guard every call to this function by a proper #ifdef in board
files.

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Igor Grinberg <grinberg@compulab.co.il>