ARM: at91: dt: at91sam9x5: add i2c-gpio pinctrl

i2c-gpio is sometimes used in place of i2c-at91.
This adds the pin muxes for the gpios.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>