raspberry pi 4: cpufreq support

The submitter notes that the bcm2835_cpufreq driver really just needs the
rpi4 compat string added to it; powerd subsequently works and the dev.cpu.0
sysctl values look sane and can be successfully manipulated.

Submitted by:	James Mintram <me@jamesrm.com>
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D25349

sys/arm/broadcom/bcm2835/bcm2835_cpufreq.c

@@ -129,6 +129,7 @@ static struct ofw_compat_data compat_data[] = {
 	{ "brcm,bcm2835",	1 },
 	{ "brcm,bcm2836",	1 },
 	{ "brcm,bcm2837",	1 },
+	{ "brcm,bcm2711",	1 },
 	{ NULL, 0 }
 };