bcm2835_clkman: add RPI4 compat string

This enables attachment on the RPI4, thus enabling the bcm2835_pwm
driver too. Per the device tree documentation, these compat strings are
equivalent, and no further changes to the driver are required.

https://lists.freebsd.org/archives/freebsd-arm/2021-June/000143.html

MFC after: 3 days