arm64: dts: fvp: Add system timer for broadcast during CPU idle

Introduce a system-level timer node in the FVP device tree to act as
a broadcast timer when CPUs are in context losing idle states where
the local timer stops on entering such low power states.

This change complements recent CPU idle state additions.

Tested-by: Leo Yan <leo.yan@arm.com>
Message-Id: <20250509154640.836093-1-sudeep.holla@arm.com>
Reviewed-by: Liviu Dudau <liviu.dudau@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>