sunxi: H6: DRAM: Add support for half DQ

Half DQ configuration seems to be very rare for H6 based boards/STBs,
but exists nevertheless. Currently the only known product which needs
this support is Tanix TX6 mini.

This commit adds support for half DQ configuration. Code was tested
for regressions on other configurations (OrangePi 3 1 GiB/LPDDR3, Tanix
TX6 4 GiB/DDR3) and none were found.

Thanks to Icenowy Zheng for help with this code.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: thomas graichen <thomas.graichen@gmail.com>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Reviewed-by: Maxime Ripard <mripard@kernel.org>