arm64: Document boot requirements for FEAT_SME_FA64

The EAC1 release of the SME specification adds the FA64 feature which
requires enablement at higher ELs before lower ELs can use it. Document
what we require from higher ELs in our boot requirements.
Signed-off-by: NMark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211026111802.12853-1-broonie@kernel.orgSigned-off-by: NWill Deacon <will@kernel.org>

Documentation/arm64/booting.rst

@@ -340,6 +340,16 @@ Before jumping into the kernel, the following conditions must be met:
     - SMCR_EL2.LEN must be initialised to the same value for all CPUs the
       kernel will execute on.
 
+  For CPUs with the Scalable Matrix Extension FA64 feature (FEAT_SME_FA64)
+
+  - If EL3 is present:
+
+    - SMCR_EL3.FA64 (bit 31) must be initialised to 0b1.
+
+ - If the kernel is entered at EL1 and EL2 is present:
+
+    - SMCR_EL2.FA64 (bit 31) must be initialised to 0b1.
+
 The requirements described above for CPU mode, caches, MMUs, architected
 timers, coherency and system registers apply to all CPUs.  All CPUs must
 enter the kernel in the same exception level.  Where the values documented