colibri_t20: add i2c support

Add I2C support in order to subsequently allow disabling the PMIC sleep
mode on low supply voltage.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>