dev-arm: Rewrite ICC_BPR0/ICC_BPR1 handling

dev-arm: Rewrite ICC_BPR0/ICC_BPR1 handling

The patch is fixing BPR reads in AA32, by removing the line

Gicv3::GroupId group =
    misc_reg == MISCREG_ICC_BPR0_EL1 ? Gicv3::G0S : Gicv3::G1S;

Where a read to ICC_BPR0 will return a G1S group.
The patch is also fixing Security banking accesses.

Change-Id: I28f1d1244c44d4b8b202d3141f8380943c7c1c86
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20620
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>