sunxi: Drop qt840a_defconfig

The qt840a is one of the many tv-boxes using the "i12" A20 pcb, but it
populates only one of the 2 places for a 16 bit dram ic, thus reducing
the buswidth to 16 bits, and the amount of ram to 512M, which is why we
had a separate config for it.

This commit switches the generic i12-tvbox_defconfig over to DRAM
autoconfiguration, so that it will work with the qt840a too, and drops the
qt840a specific config, like we've done with other memory-amount specific
configs before.

Tested on a generic i12-tvbox with 32 bit bus-width / 1G RAM, and on a
qt840a with 16 bit bus-width / 512M RAM.

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