hw/intc/arm_gic: Make ICCBPR/GICC_BPR banked

This register is banked in GICs with Security Extensions. Storing the
non-secure copy of BPR in the abpr, which is an alias to the non-secure
copy for secure access. ABPR itself is only accessible from secure state
if the GIC implements Security Extensions.

Signed-off-by: Fabian Aggeler <aggelerf@ethz.ch>
Signed-off-by: Greg Bellows <greg.bellows@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 1430502643-25909-8-git-send-email-peter.maydell@linaro.org
Message-id: 1429113742-8371-10-git-send-email-greg.bellows@linaro.org
[PMM: rewrote to fix style issues and correct handling of GICv2
 without security extensions]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
