fdt/wrappers: Introduce code to find UART DT node

fdt/wrappers: Introduce code to find UART DT node

The stdout-path property in the /chosen node of a DTB points to a device
node, which is used for boot console output.
On most (if not all) ARM based platforms this is the debug UART.
The ST platform code contains a function to parse this property and
chase down eventual aliases to learn the node offset of this UART node.

Introduce a slightly more generalised version of this ST platform function
in the generic fdt_wrappers code. This will be useful for other platforms
as well.

Change-Id: Ie6da47ace7833861b5e35fe8cba49835db3659a5
Signed-off-by: Andre Przywara <andre.przywara@arm.com>