dt: Remove dangling "select PROC_DEVICETREE"

CONFIG_PROC_DEVICETREE has been removed from the tree. This commit
removes a dangling select of the config symbol.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Paul Bolle <pebolle@tiscali.nl>
Cc: Rob Herring <rob.herring@linaro.org>