lib: utils/fdt: Fix DT property for APLIC delegation

During Linux AIA driver review, the APLIC DT property for interrupt
delegation was renamed to "riscv,delegation" so let's use the new DT
property name and fallback to old DT property name if the new DT
property name is not available.

Fixes: 34612193

 ("lib: utils/irqchip: Add FDT based driver for APLIC")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>