The children of I2C mux should be named "i2c", according to DT schema
and bindings, and they should have unit address.

This fixes dtbs_check warnings like:

  sparx5_pcb135.dtb: i2c0-imux@0: Unevaluated properties are not allowed ('#address-cells', '#size-cells', 'i2c_sfp1', 'i2c_sfp2', 'i2c_sfp3', 'i2c_sfp4' were unexpected)

and dtc W=1 warnings:

  sparx5_pcb135_board.dtsi:172.23-180.4: Warning (simple_bus_reg): /axi@600000000/sfp-eth60: missing or empty reg/ranges property
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>