irqchip/riscv-imsic: Convert to msi_create_parent_irq_domain() helper

Now that we have a concise helper to create an MSI parent domain,
switch the RISC-V letter soup over to that.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Nam Cao <tglx@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/b906a38d443577de45923b335d80fc54c5638da0.1750860131.git.namcao@linutronix.de
Link: https://lore.kernel.org/all/20241204124549.607054-6-maz@kernel.org