clk: samsung: allow building the clkout driver as module

The Exynos clock output driver can be built as module (it does not have
to be part of core init process) for better customization.  Adding a
KConfig entry allows also compile testing for build coverage.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Link: https://lore.kernel.org/r/20201110193749.261367-1-krzk@kernel.org