docs: Fix information for default TPM version

docs: Fix information for default TPM version

The current information is not accurate, because the default
is 2.0 instead of 1.2 for the tpm-crb and tpm-spapr models.

Any detailed list will surely become obsolete and out of sync
with reality over time, so let's just document that the default
model depends on a number of factors and avoid getting any more
specific than that.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Tested-by: Liu Yiding <liuyd.fnst@fujitsu.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>