arm64: Update booting requirements for GICv3 in GICv2 mode

The current requirements do not describe the case where a GICv3
system gets booted with system register access disabled, and
expect the kernel to drive GICv3 in GICv2 mode.

Describe the expected settings for that particular case.
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NMarc Zyngier <marc.zyngier@arm.com>

Documentation/arm64/booting.txt

@@ -173,13 +173,22 @@ Before jumping into the kernel, the following conditions must be met:
   the kernel image will be entered must be initialised by software at a
   higher exception level to prevent execution in an UNKNOWN state.
 
-  For systems with a GICv3 interrupt controller:
+  For systems with a GICv3 interrupt controller to be used in v3 mode:
   - If EL3 is present:
     ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1.
     ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1.
   - If the kernel is entered at EL1:
     ICC.SRE_EL2.Enable (bit 3) must be initialised to 0b1
     ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b1.
+  - The DT or ACPI tables must describe a GICv3 interrupt controller.
+
+  For systems with a GICv3 interrupt controller to be used in
+  compatibility (v2) mode:
+  - If EL3 is present:
+    ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b0.
+  - If the kernel is entered at EL1:
+    ICC_SRE_EL2.SRE (bit 0) must be initialised to 0b0.
+  - The DT or ACPI tables must describe a GICv2 interrupt controller.
 
 The requirements described above for CPU mode, caches, MMUs, architected
 timers, coherency and system registers apply to all CPUs.  All CPUs must