arm_gicv3: Fix ICC_BPR1 reset value when EL3 not implemented

If EL3 is not implemented (ie only one security state) then the
one and only ICC_BPR1 register behaves like the Non-secure
ICC_BPR1 in an EL3-present configuration. In particular, its
reset value is GIC_MIN_BPR_NS, not GIC_MIN_BPR.

Correct the erroneous reset value; this fixes a problem where
we might hit the assert added in commit a89ff39ee901.

Reported-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1496849369-30282-1-git-send-email-peter.maydell@linaro.org
