ARM: default to multi_v7_defconfig

versatile_defconfig has not been a reasonable default for a very long
time, there is very little work happening on this platform and it's
based on an old ARM926 CPU core.

This changes the default to multi_v7_defconfig, which is much more
relevant to anyone doing a build test on ARM, as it covers a wide
range of platforms.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>