sunxi: add support for R_I2C on H616

This port is needed for communication with PMIC. SPL uses it to set DRAM
voltage on H616 boards.

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