regmap: irq: Convert to use irq_domain_create_legacy()

irq_domain_create_legacy() takes a fwnode as parameter contrary to
irq_domain_add_legacy() which requires a OF node.

Switch the regmap irq domain creation to use that new function so it is not
longer limited to OF based usage.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20201030165919.86234-6-andriy.shevchenko@linux.intel.com