ARM: S3C24XX: Remove extern declaration of clk_msysclk

clk_msysclk is not needed outside of s3c2443-clock.c.

Removing the extern declaration will prevent conflicts with the
s3c2412 clk_msysclk in the following cleanups.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>