microblaze: Create the LOAD_OFFSET macro used to compute VMA vs LMA offsets

LOAD_OFFSET is the offset between the physical load address and the kernel's
virtual address.  It will be used in the upcoming commit to vmlinux.ld.S to
make sure that the LMAs of sections in vmlinux are correct.

Signed-off-by: John Williams <john.williams@petalogix.com>