Enable collectd-mod-cpufreq also for armvirt to make it
available for ipq806x devices in LEDE buildbot snapshots.
LEDE phase2 buildbot for arm_cortex-a15_neon-vfpv4 that is
the package arch for ipq806x, uses armvirt SDK instead of
ipq806x SDK.
Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>