board: riscv: qemu: increase ndev of PLIC to 1024

Increase the `ndev` of PLIC to the max of 1024 from 53, as
supported by the RISCV PLIC. The total number of IRQs is now
1035(1024 + 11), up from 64(53 + 11).

Signed-off-by: Yong Cong Sin <ycsin@meta.com>