FROMGIT: arm: Move ipi_teardown() to a CONFIG_HOTPLUG_CPU section

ipi_teardown() is only used when CONFIG_HOTPLUG_CPU is enabled.
Move the function to a location guarded by this config option.

Signed-off-by: Marc Zyngier <maz@kernel.org>

Bug: 140053385
(cherry picked from commit ac15a54e
 https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git

 irq/ipi-as-irq)
Change-Id: I9018dba7a6f7ff2a451f2405609d8d02b9bf767d
Signed-off-by: Eric Biggers <ebiggers@google.com>