clk: samsung: s3c64xx: declare s3c64xx_clk_init() in shared header

The s3c64xx_clk_init() is defined and used by the clk-s3c64xx driver and
also used in the mach-s3c64xx machine code.  Move the declaration to a
header to fix W=1 build warning:

    drivers/clk/samsung/clk-s3c64xx.c:391:13: warning: no previous prototype for 's3c64xx_clk_init' [-Wmissing-prototypes]
      391 | void __init s3c64xx_clk_init(struct device_node *np, unsigned long xtal_f,

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