DTC introduced an i2c_bus_reg check in v1.4.7, used since Linux v4.20,
which complains about upper case addresses used in the unit name.

nexys4ddr.dts names an I2C device node "ad7420@4B", leading to:

  arch/mips/boot/dts/xilfpga/nexys4ddr.dts:109.16-112.8: Warning
    (i2c_bus_reg): /i2c@10A00000/ad7420@4B: I2C bus unit address format
    error, expected "4b"

Fix this by switching to lower case addresses throughout the file, as is
*mostly* the case in the file already & fairly standard throughout the
tree.
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: stable@vger.kernel.org # v4.20+
Cc: linux-mips@vger.kernel.org