device property: Add fwnode_iomap()

This patch introduces a new helper routine - fwnode_iomap(), which
allows to map the memory mapped IO for a given device node.

This implementation does not cover the ACPI case and may be expanded
in the future. The main purpose here is to be able to develop resource
provider agnostic drivers.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Anand Ashok Dumbre <anand.ashok.dumbre@xilinx.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://lore.kernel.org/r/20211203212358.31444-2-anand.ashok.dumbre@xilinx.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>