ARM: keystone2: Rename u-boot-nand.gph to MLO

ARM: keystone2: Rename u-boot-nand.gph to MLO

NAND boot mode, ROM expects an image with a gp header in the
beginning and an 8bytes filled with zeros at the end. The same is
true for SD boot on K2G platforms but the file name should be MLO.

Renaming u-boot-nand.gph to MLO, so that same image can be used for
NAND and SD boots. And also not including all the u-boot only images
under CONFIG_SPL_BUILD.

Reported-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>