MFC r306436-r306437, r306489, r306491

r306436 by manu:
RPI-B: Add support for MULTIDELAY

100 cycles per us seems accurate enough, at least it's better than the 200 value
that was used before.

Reviewed by: andrew, imp
Differential Revision: https://reviews.freebsd.org/D8062

r306437 by manu:
RPI2: Add support for PLATFORM_SMP so we can later add it to GENERIC.

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D8063

r306489 by manu:
bcm2835_cpufreq: Only attach driver if we correcly match on the machine
compatible string.

r306491 by manu:
RPI2: Add support for MULTIDELAY, this is needed for inclusion into GENERIC.