The xloader boots the 2nd stage barebox from socfpga_barebox_part when
using NOR.  But when using MMC it boots from a hardcoded "disk0.1".
Add the mmc device name to the partition description and use it for
mmc booting.

Add an extern declaration of socfpga_barebox_part to the socfpga
header so that a board can change it to use a different partition.

Initialize socfpga_barebox_part to the default value instead of NULL to
avoid the NULL check later.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>