lib: utils/irqchip: Add FDT based driver for IMSIC

We add simple FDT irqchip driver for IMSIC so that generic platform
(and other FDT based platforms) can utilize common IMIC library.

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