sunxi: H6: Add DDR3 support to DRAM controller driver

At the moment the H6 DRAM driver only supports LPDDR3 DRAM.

Extend the driver to cover DDR3 DRAM as well.

The changes are partly motivated by looking at the ZynqMP register
documentation, partly by looking at register dumps after boot0/libdram
has initialised the controller.

Many thanks to Jernej for contributing some fixes!

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>