target-arm: Rename check_s2_startlevel to check_s2_mmu_setup

Rename check_s2_startlevel to check_s2_mmu_setup in preparation
for additional checks.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1453932970-14576-3-git-send-email-edgar.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
