arm64: protect alternatives workarounds with Kconfig options

Not all of the errata we have workarounds for apply necessarily to all
SoCs, so people compiling a kernel for one very specific SoC may not
need to patch the kernel.
Introduce a new submenu in the "Platform selection" menu to allow
people to turn off certain bugs if they are not affected. By default
all of them are enabled.
Normal users or distribution kernels shouldn't bother to deselect any
bugs here, since the alternatives framework will take care of
patching them in only if needed.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
[will: moved kconfig menu under `Kernel Features']
Signed-off-by: Will Deacon <will.deacon@arm.com>

Cc: <stable@vger.kernel.org> # v3.18.y
(cherry picked from commit c0a01b84b1fdbd98bff5bca5b201fe73fda7e9d9)
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>