MAINTAINERS: Add samsung bus frequency driver entry

This patch adds the 'BUS FREQUENCY DRIVER FOR SAMSUNG EXYNOS' entry to review the
patches as maintainer. I can access the all datasheet of Exynos SoC and test it
on some Exynos-based board. Patches will be picked up by DEVFREQ maintainer
on devfreq git repository.

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