of/unittest: Fix I2C bus unit-address error

dtc has new checks for I2C buses. Fix the warnings in unit-addresses in
the unittests.

drivers/of/unittest-data/testcases.dtb: Warning (i2c_bus_reg): /testcase-data/overlay-node/test-bus/i2c-test-bus/test-unittest14/i2c@0/test-mux-dev: I2C bus unit address format error, expected "20"
drivers/of/unittest-data/overlay_15.dtb: Warning (i2c_bus_reg): /fragment@0/__overlay__/test-unittest15/i2c@0/test-mux-dev: I2C bus unit address format error, expected "20"

Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>