ddr: marvell: a38x: add support for twin-die combined memory device

commit 6285efb8a118940877522c4c07bd7c64569b4f5f upstream.

the twin-die combined memory device should be treatened as X8
device and not as X16 one

Signed-off-by: Moti Buskila <motib@marvell.com>
Reviewed-by: Kostya Porotchkin <kostap@marvell.com>
[ - the default value for twin_die_combined is set to NOT_COMBINED for
    all boards, as this was default behaviour prior this change ]
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>