Devicetree devices encode register address ranges within the "reg"
property, with the number of cells used for addresses and for sizes
determined by the #address-cells and #size-cells properties of the
immediate parent device.
Record the number of address and size cells for each device, and
provide a dt_ioremap() function to allow drivers to map a specified
range without having to directly handle the "reg" property.
Signed-off-by: Michael Brown <mcb30@ipxe.org>