dm: core: device: add function: dev_get_driver_ops()

This commit extends the driver model device's API by function:
- dev_get_driver_ops()

And this function returns the device's driver's operations if given:
- dev pointer, is non-NULL
- dev->driver->ops pointer, is non-NULL
in other case the, the NULL pointer is returned.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>