IMA: update IMA policy documentation to include pcr= option

Commit 0260643c "ima: add policy support for extending different pcrs"
introduced a new IMA policy option "pcr=".  Missing was the documentation
for this option.  This patch updates ima_policy to include this option,
as well as an example.

Signed-off-by: Eric Richter <erichte@linux.vnet.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>