hw/arm/virt: mark timer in fdt as v8-compatible

check if the first cpu is an armv8 cpu, and if so, put
arm,armv8-timer in the compatible string list.

Note that due to this check, this patch moves the creation
of the timer fdt node to after the cpu creation loop.

Signed-off-by: Claudio Fontana <claudio.fontana@huawei.com>
Message-id: 1411736960-24206-1-git-send-email-hw.claudio@gmail.com
[PMM: updated to list arm,armv8-timer first]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
