hw/intc/arm_gic.c: Implement GICv2 GICC_DIR

The GICv2 introduces a new CPU interface register GICC_DIR, which
allows an OS to split the "priority drop" and "deactivate interrupt"
parts of interrupt completion. Implement this register.
(Note that the register is at offset 0x1000 in the CPU interface,
which means it is on a different 4K page from all the other registers.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Sergey Fedorov <serge.fdrv@gmail.com>
Message-id: 1456854176-7813-1-git-send-email-peter.maydell@linaro.org