ARM: meson: update multi_v7_defconfig

This patch updates the multi_v7_defconfig with the CONFIG_* needed by
the just added Meson anch. It also adds a new defconfig specifically for
the Meson SoCs.

Signed-off-by: Carlo Caione <carlo@caione.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>