dt-bindings: timer: Add bindings for the RISC-V timer device

We add DT bindings for a separate RISC-V timer DT node which can
be used to describe implementation specific behaviour (such as
timer interrupt not triggered during non-retentive suspend).

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20230103141102.772228-3-apatel@ventanamicro.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@kernel.org>