.mailmap: Add Mathieu Othacehe

Some contributions appears as Mathieu Othacehe and other as Mathieu
OTHACEHE.
Signed-off-by: Mathieu Othacehe <m.othacehe@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

.mailmap

@@ -123,6 +123,7 @@ Mark Brown <broonie@sirena.org.uk>
 Mark Yao <markyao0591@gmail.com> <mark.yao@rock-chips.com>
 Martin Kepplinger <martink@posteo.de> <martin.kepplinger@theobroma-systems.com>
 Martin Kepplinger <martink@posteo.de> <martin.kepplinger@ginzinger.com>
+Mathieu Othacehe <m.othacehe@gmail.com>
 Matthew Wilcox <willy@infradead.org> <matthew.r.wilcox@intel.com>
 Matthew Wilcox <willy@infradead.org> <matthew@wil.cx>
 Matthew Wilcox <willy@infradead.org> <mawilcox@linuxonhyperv.com>