drm/radeon/kms/pm: add support for SetVoltage cmd table (V2)

- This enables voltage adjustment on r6xx+ and certain
r5xx asics.
- Voltage drop support is already available for most
r1xx-r5xx asics.

V2: endian fix for voltage table.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>