arm64/mm: Document write abort detection from ESR

This patch adds an is_write_abort() wrapper and documents the detection
of the abort type on cache maintenance operations.

Cc: Will Deacon <will.deacon@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
[catalin.marinas@arm.com: only keep the is_write_abort() wrapper]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>