cpufreq: exynos: Fix driver compilation with ARCH_MULTIPLATFORM

Currently Exynos cpufreq drivers rely on globally mapped
clock controller registers to configure frequency of CPU
cores. This is obviously wrong and will be removed in near
future, but to enable support for multi-platform builds
without introducing a regression it needs to be worked
around.

This patch hacks the code to look for clock controller node
in device tree and map its registers using of_iomap(),
instead of relying on global mapping, so dependencies on
platform headers are removed and the driver can compile
again with multiplatform support.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>