lib: kconfig: Limit BINMAN_FDT for OF_SEPARATE or OF_EMBED

Generally speaking BINMAN_FDT makes sense for OF_SEPARATE or OF_EMBED.
For the other OF_CONTROL methods, it's quite possible binman node is
not available as binman is invoked during the build phase instead of
runtime. Let's only turn it on for OF_SEPARATE or OF_EMBED by default.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>