[U-Boot] Flag -mno-cache-volatile to bypass the cache

[Scott McNutt]

> Please continue to use the I/O accessor functions. Your specific
> compiler version may understand such an option, but don't expect that
> any other compiler or architecture provides the same - so better stay
> portable and use what was designed exactly for this purpose.

Yes, please do use the accessor functions. This particular compiler
flag is the result of someone's brain fart -- and does little more
than confuse the meaning of the volatile keyword (which has absolutely
_nothing_ to do with cache access) ... while promoting poorly considered
code.

--Scott