xen/arm: vpl011: Add support for vuart console in xenconsole

xen/arm: vpl011: Add support for vuart console in xenconsole

This patch finally adds the support for vuart console. It adds
two new fields in the console initialization:

- optional
- use_gnttab

optional flag tells whether the console is optional.

use_gnttab tells whether the ring buffer should be allocated using
grant table.

The VUART console is enabled ony for ARM.

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