binman: Add a library to access binman entries

SPL and TPL can access information about binman entries using link-time
symbols but this is not available in U-Boot proper. Of course it could be
made available, but the intention is to just read the device tree.

Add support for this, so that U-Boot can locate entries.

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