Documentation: correct of_platform_populate() argument list

The documentation doesn't match the actual function prototype. This is a
trivial patch to fix it.

Signed-off-by: Alison Chaiken <alison_chaiken@mentor.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>