Makefile: Add another kwb build target used on Marvell Armada-XP (AXP)

This build target now includes the SPL binary as the bin_hdr into the
kwb image. Its used on the AXP port with the mainlined DDR training code.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Luka Perkov <luka.perkov@sartura.hr>