clocksource: versatile: Adapt for Versatile AB and PB boards

The same 24MHz counter is also present on Versatile AB and PB boards, so
add the compatible string for them.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>