devicetree: add DT_INST_BUS(node_id)

This returns the node identifier for a node's bus. For example, for a
device on an I2C bus, it would return a node identifier for the I2C
bus controller.

We already have a DT_BUS() and DT_INST_BUS_LABEL(), but there hasn't
been a request or need for DT_INST_BUS() up until now, so it didn't
exist yet.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>