Widen match for CM4 compatibility

The existing test tries to find "Pi 4" in the d-t model string. This
works fine for the Model 4B ("Raspberry Pi 4 Model B Rev 1.x"), and the
400 ("Raspberry Pi 400 Rev 1.0"). However, it doesn't match the model
string on the Compute Module 4 ("Raspberry Pi Compute Module 4 Rev
1.0").

This commit uses the d-t compatible string instead and matches on
"raspberrypi,4" which matches the Pi 4 and 400 (which both have
"raspberrypi,4-model-b" in their compatible value) and the CM4
(which has "raspberrypi,4-compute-module"). Instead of matching a
sub-string it *might* be preferable to match "brcm,bcm2711" (which
again covers all the necessary platforms), but that might be
considered too "wide" a match?