lib: utils/irqchip: Add IMSIC library

We add simple IMSIC library which is independent of hardware description
format (FDT or ACPI). This IMSIC library can be used by custom OpenSBI
platform support to setup IMSIC for external interrupts.

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