ARM: defconfig: Update multi_v7 to use PREEMPT

Using CONFIG_PREEMPT as preemption model for ARMv7 systems
appear to be the most reasonable default.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>