ARM: dts: bcm2711-rpi-ds: Disable the BCM2835 STC

Although BCM2711 still includes the old BCM2835 system timer, the newer
per-core local timers are preferred because they are more efficient to
access and can generate core-specific interrupts. Make the usage clear
by disabling the driver for the old STC.

See: https://github.com/raspberrypi/firmware/issues/1702



Signed-off-by: Phil Elwell <phil@raspberrypi.com>