board: synquacer: developerbox: add myself as maintainer

Add myself as maintainer for SynQuacer Developerbox,
as I'm currently working on it.
This commit also removes Jassi from maintainer since he
no longer has a Developerbox.

Cc: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Masahisa Kojima <kojima.masahisa@socionext.com>
Acked-by: Jassi Brar <jaswinder.singh@linaro.org>