ARM: dts: bcm2712-rpi-5-b: Allow RTC to be disabled

Add a dtparam "rtc", so that "dtparam=rtc=off" can be used to disable
the Pi 5's onboard RTC.

See: https://forums.raspberrypi.com/viewtopic.php?t=361813



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