sunxi: Add support for I2C on H6 like SoCs

I2C support, especially R_I2C port, will be needed in future. Upcoming
support for H616 will need R_I2C to adjust DRAM voltage.

Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>