PM / devfreq: exynos: Remove unused exynos4/5 busfreq driver

This patch removes the unused exynos4/5 busfreq driver. Instead,
generic exynos-bus frequency driver support the all Exynos SoCs.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>