irqchip/mbigen: Don't use bus_get_dev_root() to find the parent

bus_get_dev_root() returns sp->dev_root which is set in subsys_register(),
but subsys_register() is not called by platform_bus_init().

Therefor for the platform_bus_type, bus_get_dev_root() always returns NULL.
This makes mbigen_of_create_domain() always return -ENODEV.

Don't try to retrieve the parent via bus_get_dev_root() and
unconditionally hand a NULL pointer to of_platform_device_create() to
fix this.

Fixes: fea087fc

 ("irqchip/mbigen: move to use bus_get_dev_root()")
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20240220111429.110666-1-chenjun102@huawei.com