of/fdt: fix aliases with baudrate in earlycon

Many boards use an alias in the stdout-path specification along
with console options after a colon (e.g. serial0:115200n8). When
using earlycon, this specification currently does not work. While
fdt_path_offset supports alias resolution, it does not remove the
console options by itself. Use the fdt_path_offset_namelen variant
and provide the length of the alias to enable aliases with console
options in the stdout-path.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Rob Herring <robh@kernel.org>