stand/ofw: Add ofw_path_to_handle

ofw_path_to_handle converts a path string to a phandle_t. It searches
down the path for the first device whose type matches the passed-in
string.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37556

(cherry picked from commit ed3cc2f24829e6c5827f142cfcd9729331a1106c)