docs: xen-tpmfront.txt: convert it to .rst

In order to be able to add this file to the security book,
we need first to convert it to reST.

While this is not part of any book, mark it as :orphan:, in order
to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>