[dt] Locate parent node at point of use in dt_ioremap()

We currently rely on the recursive nature of devicetree bus probing to
obtain the region cell size specification from the parent device.
This blocks the possibility of creating a standalone console device
based on /chosen/stdout-path before probing the whole bus.

Fix by using fdt_parent() to locate the parent device at the point of
use within dt_ioremap().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/devtree.h

@@ -25,9 +25,6 @@ struct dt_device {
 	struct dt_driver *driver;
 	/** Driver-private data */
 	void *priv;
-
-	/** Register cell size specification */
-	struct fdt_reg_cells regs;
 };
 
 /** A devicetree driver */