sunxi: H616: dram: fix LPDDR3 TPR6 parsing

sunxi: H616: dram: fix LPDDR3 TPR6 parsing

Allwinner's DRAM initialisation code defines a parameter named TPR6,
presumably containing some "Vref" parameter, but containing values for
*all* DRAM types. The runtime code selects one byte based on the DRAM
type used.
This selection code was wrong for LPDDR3, the value is encoded in
bits [23:16], not [15:8]. Fix that in the code, which also aligns it
with the very similar code for the A133 and A523.

Signed-off-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reported-by: Philippe Simons <simons.philippe@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Paul Kocialkowski <paulk@sys-base.io>