arm64: Kconfig: Make ARM64_PSEUDO_NMI depend on BROKEN for now

Although we merged support for pseudo-nmi using interrupt priority
masking in 5.1, we've since uncovered a number of non-trivial issues
with the implementation. Although there are patches pending to address
these problems, we're facing issues that prevent us from merging them at
this current time:

  https://lkml.kernel.org/r/1556553607-46531-1-git-send-email-julien.thierry@arm.com



For now, simply mark this optional feature as BROKEN in the hope that we
can fix things properly in the near future.

Cc: <stable@vger.kernel.org> # 5.1
Cc: Julien Thierry <julien.thierry@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>