dt-bindings: rtc: renesas,rzg3s-rtc: Document the Renesas RTCA-3 IP

Document the RTC IP (RTCA-3) available on the Renesas RZ/G3S SoC.
The RTC IP available on Renesas RZ/V2H is almost identical with the
one found on Renesas RZ/G3S (it misses the time capture functionality
which is not yet implemented on proposed driver). For this, added also a
generic compatible that will be used at the moment as fallback for both
RZ/G3S and RZ/V2H.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20241030110120.332802-5-claudiu.beznea.uj@bp.renesas.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>