target-arm: Fix TTBR selecting logic on AArch32 Stage 2 translation

Address size is 40-bit for the AArch32 stage 2 translation,
and t0sz can be negative (from -8 to 7),
so we need to adjust it to use the existing TTBR selecting logic.

Signed-off-by: Sergey Sorokin <afarallax@yandex.ru>
Message-id: 1464974151-1231644-1-git-send-email-afarallax@yandex.ru
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
