irqchip/sifive-plic: Use devm_xyz() for managed allocation

Use devm_xyz() for allocations and mappings managed by the
Linux device driver framework.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240222094006.1030709-4-apatel@ventanamicro.com