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 <[email protected]>
Tested-by: Marcel Ziswiler <[email protected]>
Tested-on: Beaver, Jetson-TK1
Tested-by: Stephen Warren <[email protected]>