sunxi: Parameterize bit delay code in H616 DRAM driver

These values are highly board specific and thus make sense to add
parameter for them. To ease adding support for new boards, let's make
them same as in vendor DRAM settings.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Acked-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>