arm: Add ARM ERRATA 782773 workaround

arm: Add ARM ERRATA 782773 workaround

This is a workaround for Errata 782773 which effects all r0, r2 and r3
revisions.

The work-around avoids the possibility of an MMU exception in the case
where a page table walk occurs immediately after a page table update
that hasn't been flushed from the L1 data cache.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Kouei Abe <kouei.abe.cp@rms.renesas.com>
Signed-off-by: Simon Horman <horms@verge.net.au>
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v2
* Reword Kconfig description
* Add some details to changelog entry