clk: renesas: r8a779a0: Make rcar_r8a779a0_cpg_clk_register() static

When compiling with clang:

    drivers/clk/renesas/r8a779a0-cpg-mssr.c:156:21: warning: no previous prototype for function 'rcar_r8a779a0_cpg_clk_register' [-Wmissing-prototypes]
    struct clk * __init rcar_r8a779a0_cpg_clk_register(struct device *dev,
			   ^
    drivers/clk/renesas/r8a779a0-cpg-mssr.c:156:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
    struct clk * __init rcar_r8a779a0_cpg_clk_register(struct device *dev,
    ^
    static

Similarly, with sparse:

    drivers/clk/renesas/r8a779a0-cpg-mssr.c:156:12: warning: symbol 'rcar_r8a779a0_cpg_clk_register' was not declared. Should it be static?

There are no users of rcar_r8a779a0_cpg_clk_register() outside this
file, so it should be static.

Fixes: 17bcc803 ("clk: renesas: cpg-mssr: Add support for R-Car V3U")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20200924111808.15358-1-geert+renesas@glider.be