arm64: dts: arm: Drop the clock-frequency property from timer nodes

Drop the clock-frequency property from the timer nodes, since it must be
configured by the boot/secure firmware.

Cc: Liviu Dudau <liviu.dudau@arm.com>
Cc: Lorenzo Pieralisi <lpieralisi@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Message-Id: <20250512101132.1743920-1-sudeep.holla@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>