dm: core: Add dev_read_enabled() to check if a device is enabled

This function allows a device's status to be read. This indicates whether
the device should be enabled or disabled.

Note: In normal operation disabled devices will not be present in the
driver-model tree.

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