arm64: fix comment in relocate_64.S

There are two typos in the comment "invalide i-cache is enabled".
We can fix it by
  invalide -> invalidate
  is       -> if

Or, if we want to match the comment to the code, we can say
"skip invalidating i-cache if disabled".

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>