sunxi: dram: Autodetect DDR3 bus width and density

In the case if the 'dram_para' struct does not specify the exact bus
width or chip density, just use a trial and error method to find a
usable configuration.

Because all the major bugs in the DRAM initialization sequence are
now hopefully fixed, it should be safe to re-initialize the DRAM
controller multiple times until we get it configured right. The
original Allwinner's boot0 bootloader also used a similar
autodetection trick.

The DDR3 spec contains the package pinout and addressing table for
different possible chip densities. It appears to be impossible to
distinguish between a single chip with 16 I/O data lines and a pair
of chips with 8 I/O data lines in the case if they provide the same
storage capacity. Because a single 16-bit chip has a higher density
than a pair of equivalent 8-bit chips, it has stricter refresh timings.
So in the case of doubt, we assume that 16-bit chips are used.
Additionally, only Allwinner A20 has all A0-A15 address lines and
can support densities up to 8192. The older Allwinner A10 and
Allwinner A13 can only support densities up to 4096.

We deliberately leave out DDR2, dual-rank configurations and the
special case of a 8-bit chip with density 8192. None of these
configurations seem to have been ever used in real devices. And no
new devices are likely to use these exotic configurations (because
only up to 2GB of RAM can be populated in any case).

This DRAM autodetection feature potentially allows to have a single
low performance fail-safe DDR3 initialiazation for a universal single
bootloader binary, which can be compatible with all Allwinner
A10/A13/A20 based devices (if the ifdefs are replaced with a runtime
SoC type detection).

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>