linker_lists: document ll_entry_ref parameters

Avoid 'make htmldocs' build warnings:

    ./include/linker_lists.h:224: warning:
    Function parameter or member '_type' not described in 'll_entry_ref'
    ./include/linker_lists.h:224: warning:
    Function parameter or member '_name' not described in 'll_entry_ref'
    ./include/linker_lists.h:224: warning:
    Function parameter or member '_list' not described in 'll_entry_ref'

Fixes: 85114435 ("linker_lists: Allow use in data structures")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>