dm: core: Add dev_read_resource() to read device resources

Add a function which reads resources from a device, such as the device
hardware address. This uses the "reg" property in the device.

Unlike other functions there is little sense in inlining this when
livetree is not being used because it has some logic in it and this would
just bloat the code size.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Tested-on: Beaver, Jetson-TK1