[ARM] fix VIPT/VIVT macro optimisations, add comments

cacheflush.h was doing:

... VIVT only stuff
... VIPT only stuff
... VIVT or VIPT stuff

which is clearly bogus - we would only ever use the "VIVT or VIPT" case
when both VIVT and VIPT are not selected.  Fix this.

Add comments to each case, including noting the impossibility of
correctly detecting the cache type of ARM926 and ARMv6 cores from
the cache type register in the "VIVT or VIPT" case.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>