lib: utils/irqchip: Use heap in PLIC, APLIC and IMSIC drivers

Let's use heap allocation in PLIC, APLIC, and IMSIC irqchip drivers
instead of using a fixed size global array.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>