irqchip/riscv-imsic-state: Create separate function for DT

irqchip/riscv-imsic-state: Create separate function for DT

While populating IMSIC global structure, many fields are initialized
using DT properties. Make the code which uses DT properties as separate
function so that it is easier to add ACPI support later. No
functionality added/changed.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Tested-by: Björn Töpel <bjorn@rivosinc.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://patch.msgid.link/20240812005929.113499-15-sunilvl@ventanamicro.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>