target/arm: Enable FEAT_EBF16 in the "max" CPU

target/arm: Enable FEAT_EBF16 in the "max" CPU

Now that we've implemented the required behaviour for FEAT_EBF16, we
can enable it for the "max" CPU type, list it in our documentation,
and delete a TODO comment about it being missing.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>