riscv: Allow to enable PREEMPT_RT.

It is really time.

riscv has all the required architecture related changes, that have been
identified over time, in order to enable PREEMPT_RT. With the recent
printk changes, the last known road block has been addressed.

Allow to enable PREEMPT_RT on riscv.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nam Cao <namcao@linutronix.de> # Visionfive 2
Link: https://lore.kernel.org/all/20240906111841.562402-4-bigeasy@linutronix.de