mailmap: add entry for AKASHI Takahiro

Akashi-san's Linaro email address in not valid anymore.

Cc: AKASHI Takahiro <akashi.tkhro@gmail.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>