MAINTAINERS: Update git repo for Renesas clock drivers

Align the clock branch name with other renesas-* branches pulled by
subsystem maintainers.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20200925110713.2652-1-geert+renesas@glider.be