dt-bindings: update riscv plic compatible string

Add the compatible string "thead,c900-plic" to the riscv plic
bindings to support allwinner d1 SOC which contains c906 core.

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Anup Patel <anup@brainfault.org>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: Rob Herring <robh@kernel.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Samuel Holland <samuel@sholland.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220130135634.1213301-2-guoren@kernel.org