irqchip/sifive-plic: Parse number of interrupts and contexts early in plic_probe()

The SiFive PLIC driver needs to know the number of interrupts and contexts
to complete initialization. Parse these details early in plic_probe() to
avoid unnecessary memory allocations and register mappings if these details
are not available.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240222094006.1030709-7-apatel@ventanamicro.com