sunxi: Parameterize "unknown feature" in H616 DRAM driver

Part of the code, previously known as "unknown feature", also doesn't
have constant values. They are derived from TPR0 parameter in vendor
DRAM code.

Let's move that code to separate function and introduce TPR0 parameter
here too, to ease adding new boards.

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>