dts: add primitive support for multi-bus in bindings

For a single bus that supports multiple protocols, e.g. I3C and I2C,
the single value "bus:" setting is no longer sufficient, as a I3C bus
cannot be matched to a device having "on-bus: I2C". This commit
extends the "bus:" setting so that it can accept a list of values.
This change allows corresponding devicetree macros to be generated
so that DT_ON_BUS() can work properly in this scenario.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>