dt-bindings: interrupt-controller: sifive,plic: Group interrupt tuples

To improve human readability and enable automatic validation, the tuples
in "interrupts-extended" properties should be grouped using angle
brackets.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/211705e74a2ce77de43d036c5dea032484119bf7.1643360419.git.geert@linux-m68k.org