target/arm: Provide cpu property for controling FEAT_LPA2

There is a Linux kernel bug present until v5.12 that prevents
booting with FEAT_LPA2 enabled.  As a workaround for TCG, allow
the feature to be disabled from -cpu max.

Since this kernel bug is present in the Fedora 31 image that
we test in avocado, disable lpa2 on the command-line.

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