dt-bindings: mfd: st,stmfx: Remove I2C unit name

Remove the I2C unit name to fix the following build warning with
'make dt_binding_check':

Warning (unit_address_vs_reg): /example-0/i2c@0: node has a unit name, but no reg or ranges property

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>