FROMGIT: ARM: Handle no IPI being registered in show_ipi_list()

As SMP-on-UP is a valid configuration on 32bit ARM, do not assume that
IPIs are populated in show_ipi_list().

Reported-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reported-by: kernelci.org bot <bot@kernelci.org>
Tested-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>

Bug: 140053385
(cherry picked from commit 22038704
 https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git

 irq/ipi-as-irq)
Change-Id: Ic34d58cf86c1cc3c233567a572fe96861dd8139f
Signed-off-by: Eric Biggers <ebiggers@google.com>