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/78c2b19218bd62653b9fb31623a42ced45f38ea6

Signed-off-by: Guo Ren <[e-mail]>
Cc: Anup Patel <[e-mail]>
Cc: Marc Zyngier <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Samuel Holland <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Tested-by: Samuel Holland <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/irqchip/irq-sifive-plic.c

···
 
 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 */