MFC r203637:
MFC r203637:

Improve checking whether an ARM VA has a valid mapping before performing cache
sync.

VIPT/PIPT caches need valid VA-PA mapping in PTE for a cache operation to
succeed (unlike VIVT). Prior to this fix pmap was using l2pte_valid() for that
check, but this is not sufficient as the function merely checks if a PTE
exists (there can be existing but _invalid_ entries in the table).

A new pmap_has_valid_mapping() routine is introduced to do this job right by
checking proper PTE flags.

Among other potential problems this cures coherency issues with L2 caches on
MV-78100.

Submitted by:	Grzegorz Bernacki, Piotr Ziecik
Reviewed, tested by:	marcel
Obtained from:	Semihalf

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