arm64/sme: Document ABI for TPIDR2 signal information

In order to allow access to TPIDR2 from signal handlers we need to add it
to the signal context, document that we are doing so.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221208-arm64-tpidr2-sig-v3-1-c77c6c8775f4@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/sme.rst

@@ -111,6 +111,9 @@ be zeroed.
 
 * Signal handlers are invoked with streaming mode and ZA disabled.
 
+* A new signal frame record TPIDR2_MAGIC is added formatted as a struct
+  tpidr2_context to allow access to TPIDR2_EL0 from signal handlers.
+
 * A new signal frame record za_context encodes the ZA register contents on
   signal delivery. [1]