Select ARM_SINGLE_ARMV7M in defconfigs of the converted ARMv7-M
platforms.

[arnd: moved the efm32_defconfig change to the patch that
originally introduces the symbol, to avoid intermittent regression]
Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>