Teach ofw_bus_parse_xref_list_alloc to be able to return the length of the parsed...

Teach ofw_bus_parse_xref_list_alloc to be able to return the length of the parsed list.

Currently, there is no easy way to know in advance how many entries a list parsed by
ofw_bus_parse_xref_list_alloc() in sys/dev/ofw/ofw_bus_subr.c has.

This patch:

* teaches the existing function about handling idx == -1 and returning how big
  the set is; then renames it as _internal;
* create a new function that asserts idx != -1, so the old API is maintained;
* add a new function that returns just the list length.

Submitted by:	Stanislav Galabov <sgalabov@gmail.com>
Differential Revision:	https://reviews.freebsd.org/D5043