clk: renesas: cpg-mssr: Use enum clk_reg_layout instead of a boolean flag

clk: renesas: cpg-mssr: Use enum clk_reg_layout instead of a boolean flag

Geert suggested defining multiple register layout variants using an enum
[1] to support further devices like R-Car V3U.  So, use enum
clk_reg_layout instead of a boolean .stbyctrl flag.  No behavioral
change.

[1] https://lore.kernel.org/linux-renesas-soc/CAMuHMdVAgN69p9FFnQdO4iHk2CHkeNaVui2Q-FOY6_BFVjQ-Nw@mail.gmail.com/

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/1599810232-29035-2-git-send-email-yoshihiro.shimoda.uh@renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>