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: Marc Zyngier <maz@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>