hw/intc: GICv3 ITS command queue framework

Added functionality to trigger ITS command queue processing on
write to CWRITE register and process each command queue entry to
identify the command type and handle commands like MAPD,MAPC,SYNC.

Signed-off-by: Shashi Mallela <shashi.mallela@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Tested-by: Neil Armstrong <narmstrong@baylibre.com>
Message-id: 20210910143951.92242-4-shashi.mallela@linaro.org
[PMM: fixed format string nit]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>