Add a man page for the cgem(4) driver.

Submitted by:	Thomas Skibo <ThomasSkibo@sbcglobal.net>
Reviewed by:	wkoszek@, Yonghyeon PYUN <pyunyh@gmail.com>


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271907 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f