arm64: Document ICC_CTLR_EL3.PMHE setting requirements

It goes without saying, but better saying it: the kernel expects
ICC_CTLR_EL3.PMHE to have the same value across all CPUs, and
for that setting not to change during the lifetime of the kernel.
Signed-off-by: NMarc Zyngier <maz@kernel.org>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/booting.rst

@@ -213,6 +213,9 @@ Before jumping into the kernel, the following conditions must be met:
 
       - ICC_SRE_EL3.Enable (bit 3) must be initialiased to 0b1.
       - ICC_SRE_EL3.SRE (bit 0) must be initialised to 0b1.
+      - ICC_CTLR_EL3.PMHE (bit 6) must be set to the same value across
+        all CPUs the kernel is executing on, and must stay constant
+        for the lifetime of the kernel.
 
   - If the kernel is entered at EL1: