tpm: Document UEFI event log quirks

There are some weird quirks when it comes to UEFI event log. Provide a
brief introduction to TPM event log mechanism and describe the quirks
and how they can be sorted out.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>