dt-bindings: interrupt-controller: Require trigger type for T-HEAD PLIC

The RISC-V PLIC specification unfortunately allows PLIC implementations
to ignore edges seen while an edge-triggered interrupt is being handled:

  Depending on the design of the device and the interrupt handler,
  in between sending an interrupt request and receiving notice of its
  handler’s completion, the gateway might either ignore additional
  matching edges or increment a counter of pending interrupts.

Like the NCEPLIC100, the T-HEAD C900 PLIC also has this behavior. Thus
it also needs to inform software about each interrupt's trigger type, so
the driver can use the right interrupt flow.

Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220630100241.35233-4-samuel@sholland.org