ARM: multi_v7_defconfig: Enable RPMSG CHAR and CTRL

These allow user-space programs to create RPMSG endpoints and have
those endpoints show up as device files. They are enabled in the
ARM64 defconfig and are useful for the same reasons here.

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>