sunxi: Use a common CONFIG_SYS_PROMPT

The CPU info is already logged during boot e.g.
   CPU:    Allwinner A20 (SUN7I)
so the prompt is just one more thing to change for each new SoC, just makes it
"sunxi#" instead.

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