spl/tpl: change banner into upper case

commit d6330064 ("spl: Add a define for SPL_TPL_PROMPT")

changes the SPL/TPL banner from upper case into lower
case. As SPL and TPL are three-letter acronyms and they
are written in upper case, change it back to upper case.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>