dt-bindings: timer: renesas: ostm: Document Renesas RZ/G2L OSTM

Document the General Timer Module(a.k.a OSTM) found on the RZ/G2L SoC.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211112184413.4391-3-biju.das.jz@bp.renesas.com


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>