clk: samsung: Add a git tree entry to MAINTAINERS

Add a missing git tree entry for drivers/clk/samsung.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>