target-arm: Add ARMCPU fields for GIC CPU i/f config

Add fields to the ARMCPU structure to allow CPU classes to
specify the configurable aspects of their GIC CPU interface.
In particular, the virtualization support allows different
values for number of list registers, priority bits and
preemption bits.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alistair Francis <alistair.francis@xilinx.com>
Message-id: 1483977924-14522-6-git-send-email-peter.maydell@linaro.org