A problem we have today is that some instances of IS_ENABLED(FOO) have
crept in to the code. This is in turn because with checkpatch.pl this is
only a warning and not an error, so they were overlooked. And looking
deeper, in the Linux kernel this pattern is allowed because
IS_ENABLED(DEFINED_FLAG) is allowed if discouraged and a quick skim of
the instances I saw in the current kernel follow this pattern. In U-Boot
however, this is not allowed, so bump to an error.

Signed-off-by: Tom Rini <trini@konsulko.com>