xen/char: dt-uart: Allow the user to give a path to the node

xen/char: dt-uart: Allow the user to give a path to the node

On some board, there is no alias to the UART. To avoid modification in
the device tree, dt-uart should also search device by path.

To distinguish an alias from a path, dt-uart will check the first character.
If it's a / then it's path, otherwise it's an alias.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>