ARM: at91: usb_a9g20 add DT i2c support

Use i2c-gpio and enable rv3029 RTC.

Enable the rtc in the sam9g20 defconfig.

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>