dt-bindings: timer: renesas, cmt: Update R-Car Gen3 CMT1 usage

The R-Car Gen3 SoCs so far come with a total for 4 on-chip CMT devices:
 - CMT0
 - CMT1
 - CMT2
 - CMT3

CMT0 includes two rather basic 32-bit timer channels. The rest of the on-chip
CMT devices support 48-bit counters and have 8 channels each.

Based on the data sheet information "CMT2/3 are exactly same as CMT1"
it seems that CMT2 and CMT3 now use the CMT1 compat string in the DTSI.

Clarify this in the DT binding documentation by describing R-Car Gen3 and
RZ/G2 CMT1 as "48-bit CMT devices".

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>