sunxi: configs/sunxi-common.h: Enable CONFIG_CMD_PART

The recent changes to config_distro_bootcmd.h require CONFIG_CMD_PART to be
defined, as the default bootcmd not uses the "part" command.

This fixes sunxi boards not booting with v2015.04-rc1.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>