irqchip/sifive-plic: Add missing thead,c900-plic match string

The thead,c900-plic has been used in opensbi to distinguish
PLIC [1]. Although PLICs have the same behaviors in Linux,
they are different hardware with some custom initializing in
firmware(opensbi).

Qute opensbi patch commit-msg by Samuel:

  The T-HEAD PLIC implementation requires setting a delegation bit
  to allow access from S-mode. Now that the T-HEAD PLIC has its own
  compatible string, set this bit automatically from the PLIC driver,
  instead of reaching into the PLIC's MMIO space from another driver.

[1]: https://github.com/riscv-software-src/opensbi/commit/78c2b19218bd62653b9fb31623a42ced45f38ea6Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Anup Patel <anup@brainfault.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Samuel Holland <samuel@sholland.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220130135634.1213301-3-guoren@kernel.org

drivers/irqchip/irq-sifive-plic.c

@@ -398,3 +398,4 @@ static int __init plic_init(struct device_node *node,
 
 IRQCHIP_DECLARE(sifive_plic, "sifive,plic-1.0.0", plic_init);
 IRQCHIP_DECLARE(riscv_plic0, "riscv,plic0", plic_init); /* for legacy systems */
+IRQCHIP_DECLARE(thead_c900_plic, "thead,c900-plic", plic_init); /* for firmware driver */