arm64: Documentation: add list of software workarounds for errata

It's not immediately obvious which hardware errata are worked around in
the Linux kernel for an arbitrary kernel tree, so add a file to keep
track of what we're working around.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>