of: add missing documentation for of_platform_populate()

15c3597d (dt/platform: allow device name to be overridden) added a
lookup parameter to of_platform_populate() but did not update the
documentation.  This patch adds the missing documentation entry.

Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Jiri Kosina <trivial@kernel.org>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>