Add iomap() fwnode operation to implement fwnode_iomap() through fwnode
operations, moving the code in fwnode_iomap() to OF framework.

Note that the IS_ENABLED(CONFIG_OF_ADDRESS) && is_of_node(fwnode) check is
needed for Sparc that has its own implementation of of_iomap anyway. Let
the pre-compiler to handle that check.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>