clk: shmobile: r8a7778: Make r8a7778_rates[] and r8a7778_divs[] static const

r8a7778_rates[] and r8a7778_divs[] are only used in clk-r8a7778.c, and
never modified.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>