common: fdt_support: Remove check for mtdparts in fdt_fixup_mtdparts

fdt_fixup_mtdparts currently does nothing when partition info is
runtime-generated or compiled-in defaults are used.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Fix nits in commit message:
Signed-off-by: Simon Glass <sjg@chromium.org>