clk: renesas: r8a77970: Add CPEX clock

Implement support for the CPEX clock on R-Car V3M.  This clock can be
selected as a clock source for CMT1 (Compare Match Timer Type 1).

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