Makefile: Add the missing dependency for spl target

Wolfgang's patch for build time improvement is bringing
out issues due to missing dependencies in the top-level
Makefile. I get errors such as the below while building
with many threads.

make[1]: /home/a0393566local/u-boot-denx/tools/mkimage: Command not found

This also allows one to do:
$ make spl/u-boot-spl.bin
if you want to build only spl

Cc: Wolfgang Denk <wd@denx.de>
Signed-off-by: Aneesh V <aneesh@ti.com>