sunxi: Fix Orangepi_mini dtb filename

The Orangepi_mini is different enough from the regular Orangepi that it needs
its own dtb, but when it got added a copy and paste error was made and it
got the same dtb filename, fix this.

While at it also add a short description of both Orangepi boards to the
defconfig files for them.

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