arm64/ptrace: Document extension of NT_ARM_TLS to cover TPIDR2_EL0

In order to allow debuggers to discover lazily saved SME state we need
to provide access to TPIDR2_EL0, we will extend the existing NT_ARM_TLS
used for TPIDR to also include TPIDR2_EL0 as the second register in the
regset.
Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220829154921.837871-3-broonie@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/sme.rst

@@ -331,6 +331,9 @@ The regset data starts with struct user_za_header, containing:
   been read if a PTRACE_GETREGSET of NT_ARM_ZA were executed for each thread
   when the coredump was generated.
 
+* The NT_ARM_TLS note will be extended to two registers, the second register
+  will contain TPIDR2_EL0 on systems that support SME and will be read as
+  zero with writes ignored otherwise.
 
 9.  System runtime configuration
 --------------------------------