docs/system/arm/: add FEAT_DoubleLock

docs/system/arm/: add FEAT_DoubleLock

We already implement FEAT_DoubleLock (see commit f94a6df5dd6a7) when
the ID registers call for it.  This feature is actually one that must
*not* be implemented in v9.0, but since our documentation lists
everything we can emulate, we should include FEAT_DoubleLock in the
list.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Message-id: 20241122225049.1617774-7-pierrick.bouvier@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: expand commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>