arch/xtensa: Make cache utilities ALWAYS_INLINE

These are tiny functions always declared as "inline" per C99, but
that's just a hint.  In practice, they tend to be (c.f. intel_asdp)
called from very early boot circumstances where main application
symbols aren't yet available.  That obviously doesn't work, or even
link.

Make them ALWAYS_INLINE.  In practice they're so small that we don't
want them called anyway just for stack space reasons.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>