Document the on-chip CMT devices included in r8a7740 and sh73a0.

Included in this patch is DT binding documentation for 32-bit CMTs
CMT0, CMT2, CMT3 and CMT4. They all contain a single channel and are
quite similar however some minor differences still exist:
 - "Counter input clock" (clock input and on-device divider)
    One example is that RCLK 1/1 is supported by CMT2, CMT3 and CMT4.
 - "Wakeup request" (supported by CMT0 and CMT2)

Because of this one unique compat string per CMT device is selected.
Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>