clk: samsung: add clock driver for external clock outputs

This adds a driver for controlling the external clock outputs of
s3c24xx architectures including the dclk muxes and dividers.

The driver at the moment only supports the legacy non-dt boards using these
clock outputs. The clock-output control itself is part of the system-controller
mainly controlled by the pinctrl drivers. So it should most likely be
integrated there for dt platforms.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>