cpufreq: Support operating-mode-v2 tables with no voltages

Summary:

The linux device tree documentation for this states that
for v1 voltages are required, but for v2 voltages are optional.

So, handle that here - if there's no regulator/supply provided
for a v1 opmode then error out; but keep it optional for v2.
Then just don't both doing any regulator calls if it's not configured.

This isn't the best/final solution - mmel@ has suggested that
this should be flipped around a bit and print warnings if
we get an opp-microvolt property but we don't have a regulator.

Subscribers: imp
Reviewed by: mmel, jrtc27, manu

Test Plan: * IPQ4018, with no voltage tables; the freq set is called appropriately.

Differential Revision: https://reviews.freebsd.org/D33140