nvindex: introduce an nvindex registry

The TCG would like to assign a NV index range to the Linux community.
Let's hence create a registry of subranges and to which open source
projects they are delegated.

For now this is only systemd.

specs/linux_tpm_nvindex_registry.md

@@ -0,0 +1,27 @@
+---
+title: Linux TPM 2.0 NV Index Registry
+version: 1
+SPDX-License-Identifier: CC0-1.0
+---
+# 🔏 Linux TPM 2.0 NV Index Registry 🗒️
+
+The Trusted Computing Group (TCG) maintains a [Registry of Reserved TPM 2.0 Handles and
+Localities](https://trustedcomputinggroup.org/resource/registry/) which assigns TPM 2.0 NV index ranges
+(among ther things, see section 2.2) to organizations (by convention only!). They have assigned the NV index
+range **0x01D10200-0x01D105FF** to the Linux community.
+
+This registry tracks assignments of subranges of this NV index range to various interested Linux
+projects. Currently, the following subranges are assigned:
+
+| Subrange             |      # | Project | Reference                                   |
+|----------------------|--------|---------|---------------------------------------------|
+|0x01D10200-0x01D10224 |     36 | systemd | https://systemd.io/TPM2_NVINDEX_ASSIGNMENTS |
+
+If you maintain a Linux Open Source project and would like to to have a subrange delegated to your project,
+please file a suitable [Pull Request](https://github.com/uapi-group/specifications/pulls).
+
+Note that while TPM 2.0 NV indexes are not quite as scarce as PCRs they still aren't free. Hence, please
+request only minimal ranges for your purposes.
+
+We will not delegate subranges to projects that aren't under an Open Source license. For NV index delegations
+to commercial projects please contact the TCG directly.