dm: core: Allow access to the device's driver_id data

When the device is created from a device tree node, it matches a compatible
string. Allow access to that string and the associated data.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>
Acked-by: Heiko Schocher <hs@denx.de>