hw/intc/arm_gic: Fix the NS view of C_BPR when C_CTRL.CBPR is 1

When C_CTRL.CBPR is 1, the Non-Secure view of C_BPR is altered:
  - A Non-Secure read of C_BPR should return the BPR value plus 1,
  saturated to 7,
  - A Non-Secure write should be ignored.

Signed-off-by: Luc MICHEL <luc.michel@git.antfield.fr>
Message-id: 20180119145756.7629-6-luc.michel@greensocs.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
[PMM: fixed comment typo]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>