tpm: Move TCG headers into a separate file

commit 97707f12 ("tpm: Support boot measurements") moved out code
from the EFI subsystem into the TPM one to support measurements when
booting with !EFI.

Those were moved directly into the TPM subsystem and in the tpm-v2.c
library. In hindsight, it would have been better to move it in new
files since the TCG2 is governed by its own spec, it's overeall cleaner
and also easier to enable certain parts of the TPM functionality.

So let's start moving the headers in a new file containing the TCG
specific bits.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>