DEVELOPERS: add Masahiro Yamada as contact for arm_foundation_v8_defconfig

Masahiro is the last person who made significant changes to this
defconfig, so let's add him as a contact for it, so that he receives
notifications of build failures and gets Cc'ed on patches.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>