dm: core: Add ofnode_read_resource()

dm: core: Add ofnode_read_resource()

We sometimes need to read a resource from an arbitrary node. In any case
for consistency we should not put the live-tree switching code in
a dev_read_...() function. Update this to suit.

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