lib: utils/irqchip: Allow multiple FDT irqchip drivers

We can have multiple FDT irqchip drivers to be probed when a RISC-V
system has different types of interrupt controller in a hierarchy.

This will be certainly the case when a RISC-V system has both
RISC-V AIA IMSIC and RISC-V AIA APLIC implemented.

We extend simple FDT irqchip framework to allow multiple FDT
irqchip drivers to be used for same RISC-V platform.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>