dm: core: Add a function to see if a device exists

dm: core: Add a function to see if a device exists

All the uclass functions for finding a device end up creating a uclass
if it doesn't exist. Add a function which instead returns NULL in this
case.

This is useful when in the 'unbind' path, since we don't want to undo
any unbinding which has already happened.

Signed-off-by: Simon Glass <sjg@chromium.org>