dt-bindings: interrupt-controller: sifive,plic: Fix number of interrupts

The number of interrupts lacks an upper bound, thus assuming one,
causing properly grouped "interrupts-extended" properties to be flagged
as an error by "make dtbs_check".

Fix this by adding the missing "maxItems", using the architectural
maximum of 15872 interrupts.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-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/f73a0aead89e1426b146c4c64f797aa035868bf0.1643360419.git.geert@linux-m68k.org