ARM: dts: at91: fix i2c-gpio node name

i2c-gpio doesn't need a reg property. Change the node names to i2c-gpio-x
as used in other dts to remove the unit-address.

Solves:
Warning (unit_address_vs_reg): Node /i2c@0 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /i2c@1 has a unit name, but no reg property
Warning (unit_address_vs_reg): Node /i2c@2 has a unit name, but no reg property

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>