clk: samsung: s3c24xx: declare s3c24xx_common_clk_init() in shared header

The s3c2410_common_clk_init() and others are defined and used by the
clk-s3c24xx driver and also used in the mach-s3c24xx machine code.  Move
the declaration to a header to fix W=1 build warnings:

    drivers/clk/samsung/clk-s3c2410.c:320:13: warning: no previous prototype for 's3c2410_common_clk_init' [-Wmissing-prototypes]
      320 | void __init s3c2410_common_clk_init(struct device_node *np, unsigned long xti_f,
    drivers/clk/samsung/clk-s3c2412.c:205:13: warning: no previous prototype for 's3c2412_common_clk_init' [-Wmissing-prototypes]
      205 | void __init s3c2412_common_clk_init(struct device_node *np, unsigned long xti_f,
    drivers/clk/samsung/clk-s3c2443.c:341:13: warning: no previous prototype for 's3c2443_common_clk_init' [-Wmissing-prototypes]
      341 | void __init s3c2443_common_clk_init(struct device_node *np, unsigned long xti_f,

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>