sunxi: Parameterize some of H616 DDR3 timings

Currently twr2rd, trd2wr and twtp are constants, but according to
vendor driver they are calculated from other values. Do that here too,
in preparation for later introduction of new parameter.

While at it, introduce constant for t_wr_lat, which was incorrectly
calculated from tcl before.

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