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

[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>