dm: core: Add device-based 'read' functions to access DT

dm: core: Add device-based 'read' functions to access DT

It is common to read a device-tree property from the node associated with
a device. Add convenience functions to do this so that drivers do not need
to deal with accessing the ofnode from the device.

These functions all start with 'dev_read_' to provide consistent naming
for all functions which read information from a device's device tree node.

These are inlined when using the flat DT to save code size. The live tree
implementation is added in a later commit.

Signed-off-by: Simon Glass <sjg@chromium.org>