clocksource: arm_global_timer: Only check for unusable timer on A9

The check for a usable global timer in the probe code does not enquire
which CPU we are currently running on. This can cause the driver to
incorrectly assume we have an unusable global timer if we are running
on a CPU other than A9.

Before checking the CPU revision, ensure we are running on an A9 CPU.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>