irqchip/riscv-aplic: Prevent crash when MSI domain is missing

If the APLIC driver is probed before the IMSIC driver, the parent MSI
domain will be missing, which causes a NULL pointer dereference in
msi_create_device_irq_domain().

Avoid this by deferring probe until the parent MSI domain is available. Use
dev_err_probe() to avoid printing an error message when returning
-EPROBE_DEFER.

Fixes: ca8df97f ("irqchip/riscv-aplic: Add support for MSI-mode")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241114200133.3069460-1-samuel.holland@sifive.com