clk: renesas: r8a77995: Add missing CPEX clock

The R-Car Gen3 HardWare Manual Errata for Rev. 0.80 (Feb 28, 2018) added
the CPEX clock on R-Car D3.  This clock can be selected as a clock
source for CMT1 (Compare Match Timer Type 1).

Add the missing clock to the DT bindings header, and implement support
for it in the clock driver.

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