lmb: Improve kernel-doc comments

lmb: Improve kernel-doc comments

Fix warnings from kernel-doc script. Improve and unify overall style of
kernel-doc comments in lmb source files. Move all kernel-doc comments
for public functions into the header, as recommended in U-Boot
documentation [1]:

    Non-trivial functions should have a comment which describes what
    they do. If it is an exported function, put the comment in the
    header file so the API is in one place. If it is a static function,
    put it in the C file.

This also takes care of existing duplication. While at it, do a bit of
cosmetic cleanups as well.

No functional change.

[1] doc/develop/codingstyle.rst

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>