xen/arm: vpl011: Add a new vuart node in the xenstore

xen/arm: vpl011: Add a new vuart node in the xenstore

Add a new vuart console node to xenstore. This node is added at

/local/domain/$DOMID/vuart/0.

The node contains information such as the ring-ref, event channel,
buffer limit and type of console.

Xenconsole reads the node information to setup the ring buffer and
event channel for sending/receiving vuart data.

Signed-off-by: Bhupinder Thakur <bhupinder.thakur@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>