irqchip/sifive-plic: Convert PLIC driver into a platform driver

The PLIC driver does not require very early initialization so convert
it into a platform driver.

After conversion, the PLIC driver is probed after CPUs are brought-up
so setup cpuhp state after context handler of all online CPUs are
initialized otherwise PLIC driver crashes for platforms with multiple
PLIC instances.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240222094006.1030709-2-apatel@ventanamicro.com