drm/amd/pm: support overdrive vddgfx offset setting(V2)

This is supported by Sienna Cichlid, Navy Flounder and Dimgrey
Cavefish. For these ASICs, the target voltage calculation can be
illustrated by "voltage = voltage calculated from v/f curve +
overdrive vddgfx offset".

V2: limit the smu_version check for Sienna Cichlid only

Here are some sample usages about this new OD setting:
1. Check current vddgfx offset setting by
cat /sys/class/drm/card0/device/pp_od_clk_voltage
...
...
OD_VDDGFX_OFFSET:
0mV
...
...

2. Set new vddgfx offset by
echo "vo 10" > /sys/class/drm/card0/device/pp_od_clk_voltage
cat /sys/class/drm/card0/device/pp_od_clk_voltage
...
...
OD_VDDGFX_OFFSET:
10mV
...
...
3. Commit the new setting by
echo "c" > /sys/class/drm/card0/device/pp_od_clk_voltage

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>