fdt: remove fdtdec_get_alias_node() function

The fdt_path_offset() checks an alias too.

fdtdec_get_alias_node(blob, "foo") is equivalent to
fdt_path_offset(blob, "foo").

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>