irqchip/sifive-plic: Improve locking safety by using irqsave/irqrestore

Now that PLIC driver is probed as a regular platform driver, the lock
dependency validator complains about the safety of handler->enable_lock
usage:

[    0.956775]  Possible interrupt unsafe locking scenario:

[    0.956998]        CPU0                    CPU1
[    0.957247]        ----                    ----
[    0.957439]   lock(&handler->enable_lock);
[    0.957607]                                local_irq_disable();
[    0.957793]                                lock(&irq_desc_lock_class);
[    0.958021]                                lock(&handler->enable_lock);
[    0.958246]   <Interrupt>
[    0.958342]     lock(&irq_desc_lock_class);
[    0.958501]
                *** DEADLOCK ***

To address above, use raw_spin_lock_irqsave/unlock_irqrestore() instead
of raw_spin_lock/unlock().

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240222094006.1030709-8-apatel@ventanamicro.com