arm64: Cope with CPUs stuck in VHE mode

arm64: Cope with CPUs stuck in VHE mode

It seems that the CPUs part of the SoC known as Apple M1 have the
terrible habit of being stuck with HCR_EL2.E2H==1, in violation
of the architecture.

Try and work around this deplorable state of affairs by detecting
the stuck bit early and short-circuit the nVHE dance. Additional
filtering code ensures that attempts at switching to nVHE from
the command-line are also ignored.

It is still unknown whether there are many more such nuggets
to be found...

Reported-by: Hector Martin <marcan@marcan.st>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210408131010.1109027-3-maz@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>