ARM: defconfig: enable cpufreq driver for RPi

This enables on both multi_v7_defconfig and bcm2835_defconfig the new
firmware based clock and cpufreq drivers for the Raspberry Pi platform.

In the case of bcm2835_defconfig, as the cpufreq subsystem was disabled,
the conservative governor was selected as default since it better
handles the high frequency transition latency.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>