Now that we can execute a CONFIG_SMP kernel on a uniprocessor system,
extra care has to be taken in the PMU IRQ affinity setting code to
ensure that we don't always fail to initialise.

This patch changes the CPU PMU initialisation code so that when we
only have a single IRQ, whose affinity can not be changed at the
controller, we report success (0) rather than -EINVAL.
Reported-by: NAvik Sil <avik.sil@linaro.org>
Acked-by: NJamie Iles <jamie@jamieiles.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NRussell King <rmk+kernel@arm.linux.org.uk>