target-arm: implement PD0/PD1 bits for TTBCR

Corrected handling of writes to TTBCR for ARMv8 (previously UNK/SBZP
bits are not RES0) and ARMv7 (new bits PD0/PD1 for CPUs with Security
Extensions).

Bits PD0/PD1 are now respected in get_phys_addr_v6/v5() and
get_level1_table_address.

Signed-off-by: Fabian Aggeler <aggelerf@ethz.ch>
Message-id: 1402409556-18574-1-git-send-email-aggelerf@ethz.ch
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
