ARM: configs: fix duplicate entry in multi_v7

Two of the recent patches each added support for CPU_FREQ, which
causes a kconfig warning:

arch/arm/configs/multi_v7_defconfig:443:warning: override: reassigning to symbol CPU_FREQ

This removes one of the two in order to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>