arm64: Document the requirement for SCR_EL3.HCE

It is amazing that we never documented this absolutely basic
requirement: if you boot the kernel at EL2, you'd better
enable the HVC instruction from EL3.

Really, just do it.
Signed-off-by: Marc Zyngier <maz@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/r/20210812190213.2601506-6-maz@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/booting.rst

@@ -207,10 +207,17 @@ Before jumping into the kernel, the following conditions must be met:
   software at a higher exception level to prevent execution in an UNKNOWN
   state.
 
-  - SCR_EL3.FIQ must have the same value across all CPUs the kernel is
-    executing on.
-  - The value of SCR_EL3.FIQ must be the same as the one present at boot
-    time whenever the kernel is executing.
+  For all systems:
+  - If EL3 is present:
+
+    - SCR_EL3.FIQ must have the same value across all CPUs the kernel is
+      executing on.
+    - The value of SCR_EL3.FIQ must be the same as the one present at boot
+      time whenever the kernel is executing.
+
+  - If EL3 is present and the kernel is entered at EL2:
+
+    - SCR_EL3.HCE (bit 8) must be initialised to 0b1.
 
   For systems with a GICv3 interrupt controller to be used in v3 mode:
   - If EL3 is present: